For the Scavenger we require a first pass over global handles for identifying
unmodified nodes because the Scavenger might have already written forwarding
pointers during scanning, making it hard to perform the proper checks.

The minor MC does not mutate the object graph during marking and can thus merge
this phase into the regular phase executed during marking roots.

Furthermore, moves processing into the parallel marking phase of the minor MC
collector.

Bug: chromium:720477, chromium:651354
Change-Id: Id33552124264e3ab0bdf34d22ac30c19c1522707
Reviewed-on: https://chromium-review.googlesource.com/509550
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45461}