Add DCHECK in IndexedDBDispatcher::ThreadSpecificInstance to ensure

      IndexedDBDispatcher doesn't get re-created during worker shutdown.

This could happen if there are IDB objects that survive the call to
didStopWorkerRunLoop.

This CL also makes RenderThreadImpl call new IndexedDBDispatcher
instead of IndexedDBDispatcher::ThreadSpecificInstance to avoid
hitting that DCHECK in tests.  WebRTCAudioDeviceTest creates a
RenderThreadImpl object, deletes it, and creates another render
thread object, all on the same thread.

BUG=121734
TEST=


Review URL: http://codereview.chromium.org/10052005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@132128 0039d316-1c4b-4281-b951-d872f2087c98