Fix IndexedDB race condition during shutdown.

      Make sure IndexedDBContext is deleted before returning from
~WebKitContext. Otherwise, UtilityProcessHost may still be in batch mode
when the IO thread is shut down.

BUG=67422
TEST=browser_tests --gtest_filter=IndexedDBBrowserTest.*

Review URL: http://codereview.chromium.org/6209005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@72157 0039d316-1c4b-4281-b951-d872f2087c98