Number of small improvements to CleanupReference

- sRef is now only accessed on main thread, so CleanupReference should
  now be threadsafe
- synchronously flushes sQueue when references added or removed on main
  thread, to avoid falling behind when heap is churning rapidly (i.e.
  finalizers never getting to run scenario).
- ensure the referene gets removed from sRefs after a GC as otherwise
  it might have remained on there indefinitely.

BUG=


Review URL: https://chromiumcodereview.appspot.com/11648040

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@174931 0039d316-1c4b-4281-b951-d872f2087c98
1 file changed