Revert 150232 (speculative, probably broke

Revert 150232 (speculative, probably broke
LauncherFaviconLoaderBrowsertest.ChangeLauncherIcons
on cros bots) - Update favicon URLs on FinishLoad for main frame

Previously it was done on didStopLoading for _any_ frame.
That causes a stream of spurious favicon URL updates for
every <iframe>.
This is now moved to on didFinishLoad and get triggered only
for the main frame.  This ensures that icon loads always get
initiated after all of the other page resources have been
fetched.  Note that DidFinishDocumentLoad runs potentially
before all subresources have been fetched, which means that
the icon requests may compete with subresources for network
bandwidth.  didFinishLoad is the closest place to where
IconController does it, and we already have icons at this point.

BUG=131567
Review URL: https://chromiumcodereview.appspot.com/10831163

TBR=aruslan@chromium.org
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=150277