This is a reland of 35c923cc
Original change's description:
> [heap] Add support for atomic access to page flags.
> 
> This patch renames AsAtomicWord to AsAtomicPointer and
> adds new AsAtomicWord that works with intptr_t.
> 
> Slot recording uses atomic page flag accessors.
> 
> BUG=chromium:694255
> 
> Change-Id: I1c692813244b41320182e9eea50462d1802fcd98
> Reviewed-on: https://chromium-review.googlesource.com/597688
> Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
> Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#47086}

Bug: chromium:694255
Change-Id: I36780ff4001e068815d4be1e16cd06f1a4f98d13
Reviewed-on: https://chromium-review.googlesource.com/599909Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47131}