Web Request: Introduce onActionIgnored event for conflicts.

This CL introduces the onActionIgnored event on the web request API which is
fired when an extension's proposed modification to a network request is ignored.
This happens in case of conflicts with other extensions. This should allow the
extensions to address conflicts as they like.

Also, we no longer notify the user in case of conflicts between extensions.

BUG=111700

Change-Id: I3e96740aee6b79b0823545bf17b4a50bbc8758aa
Reviewed-on: https://chromium-review.googlesource.com/1150009
Commit-Queue: Karan Bhatia <karandeepb@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578728}
22 files changed