Correctly handle NULL ResourceDispatcherHostImpl::scheduler()

During shutdown, the ResourceDispatcherHost has cleared its scheduler after it
stops handling renderer requests. However there may still be renderers alive at
this point, and messages in their pipes, so make sure handling these messages
don't try to access the destroyed ResourceScheduler.

BUG=335578

Review URL: https://codereview.chromium.org/142133002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245948 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 245948