This CL provides the basic infrastructure on the GC side for write protected code pages.

The only thing missing on the GC side is the out-of-line free list implementation. In this
CL sweeper threads and the mutator need to synchronize when page protection mode changes.
This would not be necessary if the sweepr use and out-of-line free list.

Code allocation is currently protected by a CodeSpaceMemoryModificationScope. This may
go away with a unification of code space allocation and initialization that will happen
later.

One thing missing in this CL: freshly added pages are still read+write+executable. This
also needs to change: WIP

Bug: chromium:774108,v8:6792
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: Ib8d1ed1c09cc144791e462277bf43a4641e1490d
Reviewed-on: https://chromium-review.googlesource.com/716379
Commit-Queue: Hannes Payer <hpayer@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48774}