Remove DocumentLoader::m_isStopping

It appears to be unnecessary and/or duplicate information.

Its primary purpose was to prevent an infinite recursion that occurred in
Safari (http://src.chromium.org/viewvc/blink/?revision=104593&view=revision).

Given that this crash appeared to involve some interaction with Safari's
appcache implementation (which is very different from ours), I'm inclined
to believe this re-entrancy concern doesn't apply to us.

BUG=none

Review URL: https://chromiumcodereview.appspot.com/22907006

git-svn-id: svn://svn.chromium.org/blink/trunk@156326 bbb929c8-8fbe-4397-9dbb-9b2b20218538