New code should use nullptr instead of NULL.

This patch updates existing use of NULL to nullptr where applicable,
making the code base more consistent.

BUG=v8:6928,v8:6921

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng;master.tryserver.v8:v8_linux_noi18n_rel_ng
Change-Id: I4687f5b96fcfd88b41fa970a2b937b4f6538777c
Reviewed-on: https://chromium-review.googlesource.com/718338
Commit-Queue: Mathias Bynens <mathias@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48557}

This is a reland of bb881c88
Original change's description:
> Document RootIndexMap's requirement for immovable roots.
> 
> R=mstarzinger@chromium.org
> 
> Change-Id: I27cf279c77204becc449221e1bcebeacbc275792
> Reviewed-on: https://chromium-review.googlesource.com/565281
> Commit-Queue: Yang Guo <yangguo@chromium.org>
> Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#46517}

Change-Id: I6eeb68cee3bd2dff0eb6696bbe43a4d602ab960d
Reviewed-on: https://chromium-review.googlesource.com/566458Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46548}

This reverts commit bb881c88.

Reason for revert: nosnap debug:
https://build.chromium.org/p/client.v8/builders/V8%20Linux%20-%20nosnap%20-%20debug/builds/13930

Original change's description:
> Document RootIndexMap's requirement for immovable roots.
> 
> R=​mstarzinger@chromium.org
> 
> Change-Id: I27cf279c77204becc449221e1bcebeacbc275792
> Reviewed-on: https://chromium-review.googlesource.com/565281
> Commit-Queue: Yang Guo <yangguo@chromium.org>
> Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#46517}

TBR=yangguo@chromium.org,mstarzinger@chromium.org

Change-Id: Ifac3ec6192207bf2be8932408c37bea66bb435e8
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/565598Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46522}

R=mstarzinger@chromium.org

Change-Id: I27cf279c77204becc449221e1bcebeacbc275792
Reviewed-on: https://chromium-review.googlesource.com/565281
Commit-Queue: Yang Guo <yangguo@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46517}

Changes include:
 - Adding V8_EXPORT macro for SnapshotCreator
 - Removing outdated DCHECKs.
 - Allow nullptr as external reference. This required a...
 - Refactoring of hashmaps used by the serializer.
 - Remove external references for counters. These are not used
   anywhere for isolates that are being serialized.
 - Put template infos into the partial snapshot cache.
 - Remove unnecessary presubmit check for external references.
   mksnapshot crashes if external references are missing.

R=jochen@chromium.org, vogelheim@chromium.org
BUG=chromium:617892

Review-Url: https://codereview.chromium.org/2490783004
Cr-Commit-Position: refs/heads/master@{#40949}

Make MatchFun a template parameter in TemplateHashMapImpl, moving the
PointersMatch function down to an implementation which extends
TemplateHashMapImpl to void* key and value (i.e. the same as the current
HashMap and ZoneHashMap typedefs).

This will allow other instantiations of TemplateHashMapImpl, with
different MatchFun values, e.g. std::equal_to, to have their key
equality test inlined, rather than calling a function pointer,

Review-Url: https://codereview.chromium.org/2354593002
Cr-Commit-Position: refs/heads/master@{#39868}

We ported hashmap.h into libsampler as a workaround before, so the main focus of
this patch is to reduce code duplication. This patch moves the hashmap into
src/base as well as creates DefaultAllocationPolicy using malloc and free.

BUG=v8:5050
LOG=n

Review-Url: https://codereview.chromium.org/2010243003
Cr-Commit-Position: refs/heads/master@{#36873}

The serializer collects objects in iteration order, not in allocation
order. This means that the deserializer will put these objects in
iteration order onto the reserved pages as well. There is no guarantee
that objects that were on the first page will end up on the first page
after deserialization.

Until now we got lucky, since we only ever need one space per page for
the default snapshot. For roots, the iteration order and allocation
order also do not differ enough to cause any issue for immortal
immovable root objects. These objects need to stay on the first page of
its allocated space to not move.

However, let's make sure it stays this way, and we realize soon enough
if this assumption does not hold.

R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/1675553002

Cr-Commit-Position: refs/heads/master@{#33810}

This is in preparation of using it elsewhere.

R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/1408163003

Cr-Commit-Position: refs/heads/master@{#31499}