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This CL let iOS SearchEngineTabHelper extend FaviconDriverObserver so that
it has equivalent functionality as the SearchEngineTabHelper in
/chrome/browser/ui/search_engines.

Bug: 433824
Change-Id: If49d1b46d90b2250c2b378d77d9e3cea5cdd96a5
Reviewed-on: https://chromium-review.googlesource.com/c/1329171
Commit-Queue: Yi Su <mrsuyi@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606898}