[heap] Simplify MarkingDeque implementation.

[heap] Simplify MarkingDeque implementation.

This removes the dependency of the MarkingDeque on the underlying
collector by moving out the live bytes accounting and object color
mutations into a wrapper. The MarkingDeque can now be split into a
separate file.

R=hpayer@chromium.org

Review URL: https://codereview.chromium.org/1293773002

Cr-Commit-Position: refs/heads/master@{#30194}