Fixed IndexedDBCursor destruction bug.
IndexedDBCursor's can now be destroyed with cursor messages still in an IPC
message queue.
Specifically in 368134 IndexedDBCallbacks::OnSuccessWithPrefetch() was called,
enqueuing a CallbacksSuccessCursorPrefetch message via IndexedDBDispatcher.
Before that message was handled (by IndexedDBDispatcher::OnSuccessCursorPrefetch())
IndexedDBDispatcher::CursorDestroyed() was called which removing the cursor ID
from the map. This new behavior started as a result of Blink's r172568 which
began deleting cursors earlier than the previous implementation.
Also, unrevert's r266866 to re-enable the RenderThreadShutdownTest test.
BUG=368134
Review URL: https://codereview.chromium.org/
260713009
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@267571 0039d316-1c4b-4281-b951-d872f2087c98