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Change notification NOTIFICATION_FAVICON_UPDATED into an event sent to
FaviconDriverObserver and change client code to be observers instead.

Pass the FaviconDriver to OnFaviconUpdated() event so that listeners can
find the content::WebContents corresponding to the event (as some of the
client code did listen to multiple content::WebContents).

BUG=362072

Review URL: https://codereview.chromium.org/1059743003

Cr-Commit-Position: refs/heads/master@{#324201}