In order to support conservative stack scanning, we need to be able to
pin pages if they're pointed to by an ambiguous pointer. This CL lets us
do this by providing an IS_PINNED flag which, when enabled, prevents a
page from being selected as an evacuation candidate during compaction.

Bug: v8:10614
Change-Id: I45d1c74b83aad1b56e078cc270749a0aee804716
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2278465
Commit-Queue: Jake Hughes <jakehughes@google.com>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68815}