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
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.