Fix TestGCScope in blink_heap_unittests so that it leaves freelists alone.

Fix TestGCScope in blink_heap_unittests so that it leaves freelists alone.

The TestGCScope only needs to stop and resume threads. It should not
change the state of the heap. The old code made the heap consistent
for scanning which cleared all freelists and never rebuild them.

R=haraken@chromium.org, erik.corry@gmail.com, wibling@chromium.org
BUG=

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=165109

[Mads Ager (chromium), 2014-01-14 17:44:42]

This was uncovered by the tests that Gustav is moving over in one of his
outstanding changelists.

[haraken, 2014-01-15 00:22:09]

LGTM

[commit-bot: I haz the power, 2014-01-15 06:40:41]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/ager@chromium.org/138273003/1

[Mads Ager (chromium), 2014-01-15 06:58:29]

Committed patchset #1 manually as r165109 (presubmit successful).