The WebThreadedDataReceiver will now check if the parserthread is still alive before returning it to the ThreadedDataProvider

This fixes a crash when a renderer is shut down while a request is still alive, due to the ResourceDispatcher being shut down after Blink is.

R=abarth
BUG=386383

Review URL: https://codereview.chromium.org/345663002

git-svn-id: svn://svn.chromium.org/blink/trunk@176487 bbb929c8-8fbe-4397-9dbb-9b2b20218538