content: Remove an implicit assumption of resource_dispatcher.cc.
The implementation of ResourceDispatcher::OnMessageReceived() assumed that a
call to FlushDeferredMessages(request_id) would not invalidate the iterator
obtained from GetPendingRequestInfo(request_id). As far as I can tell, this
assumption relies on implicit behavior of consumers of ResourceDispatcher. I've
removed this assumption by adding a second call to
GetPendingRequestInfo(request_id) following the call to
FlushDeferredMessages(request_id).
BUG=527588
Review URL: https://codereview.chromium.org/
1306283007
Cr-Commit-Position: refs/heads/master@{#348054}