Adds a scavenge GC pass that collects unmodified references instead of
processing object groups. This mode can be controlled by setting
FLAG_scavenge_reclaim_unmodified_objects. By default this is turned off.
Also, modified a test case to suit the handle the new GC pass.

BUG=v8:4421
LOG=N

Review URL: https://codereview.chromium.org/1410593005

Cr-Commit-Position: refs/heads/master@{#31599}