The IsWhite check in the write barrier uses word size memory operations.
It should use 32-bit cell size operation instead.

Bug: v8:6955
Change-Id: I5bbcd99dcd7e3d435f96022a745a6c80c83eb3b3
Reviewed-on: https://chromium-review.googlesource.com/735153Reviewed-by: Hannes Payer <hpayer@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48910}