Deal with canceled requests when flushing deferred messages.

Flushing deferred messages might lead to a request being canceled
(e.g. when an ImageResource loads a corrupt image). The code didn't
fully take this into account which would cause crashes (and resource
leaks if it would have survived).

BUG=

Review-Url: https://codereview.chromium.org/2425173003
Cr-Commit-Position: refs/heads/master@{#427298}
2 files changed