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Add a method to ExtensionPrefsObserver to monitor for runtime permission
changes, and listen for these changes in the chrome://extensions page.
This solves an issue where adding a new runtime host permission wouldn't
be reflected immediately in the chrome://extensions page if it wasn't a
permission directly requested by the extension, because there wouldn't
be an active permissions change.

Also reorder setting the "withhold permissions" pref to happen before
adding the withheld permissions so that when the observers are notified,
the bit is properly set.

Bug: 878939

Change-Id: I98c70ecd951a6ebb3d3372fec797f3657215a2b1
Reviewed-on: https://chromium-review.googlesource.com/1196107
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: Karan Bhatia <karandeepb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589081}