This patch merges ObjectMarking and MarkingState. The new marking state
encapsulates object marking, live byte tracking, and access atomicity.

The old ObjectMarking calls are now replaced with calls to marking
state. For example:
ObjectMarking::WhiteToGrey<kAtomicity>(obj, marking_state(obj)
becomes
marking_state()->WhiteToGrey(obj)

This simplifies custom handling of live bytes and allows to chose
atomicity of markbit accesses depending on collector's state.

This also decouples marking bitmap from the marking code, which will
allows in future to use different data-structure for mark-bits.

Bug: chromium:694255
Change-Id: Ifb4bc0144187bac1c08f6bc74a9d5c618fe77740
Reviewed-on: https://chromium-review.googlesource.com/602132
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47288}