WebLeakDetector: Fix false positive leaks on worker tests when oilpan is enabled.

This patch adds another round of GC to make sure Document is destroyed after |WorkerMessagingProxy::workerObjectDestroyed|.

BUG=389042

Review URL: https://codereview.chromium.org/350013003

git-svn-id: svn://svn.chromium.org/blink/trunk@177043 bbb929c8-8fbe-4397-9dbb-9b2b20218538