[heap] Make the current GCCallbackFlags are part of {Heap}.

[heap] Make the current GCCallbackFlags are part of {Heap}.

Moves the GCCallbackflags where they belong, i.e., {Heap}, and gets rid of
IncrementalMarking::Start() callsites.

BUG=

Review URL: https://codereview.chromium.org/1314853002

Cr-Commit-Position: refs/heads/master@{#30349}