[V8][Chromium] Race between worker accessing WebDatabase and frame closing down.

[V8][Chromium] Race between worker accessing WebDatabase and frame closing down.
https://bugs.webkit.org/show_bug.cgi?id=78150
Handle case when the frame is shut down under us gracefully.

Patch by Dmitry Lomov <dslomov@chromium.org> on 2012-02-08
Reviewed by David Levin.

* src/DatabaseObserver.cpp:
(WebCore::DatabaseObserver::canEstablishDatabase): Handle WebWorkerBase::view returning 0.
* src/IDBFactoryBackendProxy.cpp:
(WebKit::IDBFactoryBackendProxy::openFromWorker): Handle WebWorkerBase::view returning 0.
* src/WebWorkerClientImpl.cpp:
(WebKit::WebWorkerClientImpl::allowFileSystem):
(WebKit::WebWorkerClientImpl::allowDatabase):
(WebKit::WebWorkerClientImpl::view):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@107174 268f45cc-cd09-0410-ab3c-d52691b4dbfc