Fix mark stack expand race.

We now guard parallel mark stack pushing with a lock. This is
only used by checkpoint root marking. I did not observe a
significant slowdown by looking at ritzperf and maps, but it may
be worth reinvestigating in the future.

Also a bit of refactoring.

Bug: 10113123

Change-Id: Ifcb12d14df437e2aea9a1165a9568054f80d91b3
3 files changed