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}