This reverts commit 35c923cc.

Reason for revert: speculative revert for GC stress failure

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}

TBR=ulan@chromium.org,mlippautz@chromium.org

Change-Id: Id77ce7970c54a55646c072787e88311f6f3e6e91
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: chromium:694255
Reviewed-on: https://chromium-review.googlesource.com/598967Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47094}