Revert "[heap] Make non-atomic markbit operations consistent with atomic ones."

This reverts commit dd37366f

Revert "[heap] Use atomic marking operations in incremental marking if"

This reverts commit 1f2c3596

Revert "[heap] Prepare IncrementalMarking::VisitObject for concurrent marking."

This reverts commit 00d1e2cf

Revert "[heap] Use shared markbits in the concurrent marker."

This reverts commit b0db0541

https://codereview.chromium.org/2857713002 blocks the current roll:
https://codereview.chromium.org/2857423002/

Doesn't revert cleanly.

NOTRY=true
TBR=ulan@chromium.org

Bug: chromium:694255
Change-Id: Iada35af5c2529cd9e604802700604b16cc30aa2d
Reviewed-on: https://chromium-review.googlesource.com/497387Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45118}