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=

Review URL: https://codereview.chromium.org/138273003

git-svn-id: svn://svn.chromium.org/blink/trunk@165109 bbb929c8-8fbe-4397-9dbb-9b2b20218538