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}

ZoneList still used List as a base class, so this CL merges the two
classes together. We also remove unused functions in List and ZoneList.

We keep the inline header but move it to src/zone/zone-list-inl.h. The
includes that use this header are still quite tangled, but we can fix
that later.

Bug: v8:6333
Cq-Include-Trybots: master.tryserver.v8:v8_linux_noi18n_rel_ng
Change-Id: Ia809813834b2328ff616623f8a843812a1eb42a7
Reviewed-on: https://chromium-review.googlesource.com/681658
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48200}