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Split FaviconTabHelper into three classes depending on what can be
shared with iOS (FaviconDriverImpl), what only depends on //content
(ContentFaviconDriver) and what depends on //chrome (FaviconTabHelper).

FaviconTabHelper still exists to provide an easier factory (as
ContentFaviconDriver cannot access KeyedService factories defined in
//chrome) as FaviconTabHelper::CreateForWebContents() is called from
multiple locations in //chrome code and to provide an implementation for
FaviconTabHelper::ShouldDisplayFavicon (this method will be turned into
a free function in a followup CL as it only depends on WebContents
public interface, not on ContentFaviconDriver).

Introduce WebFaviconDriver, an iOS implementation of FaviconDriver that
uses FaviconDriverImpl to share as much code as possible with the
ContentFaviconDriver.

BUG=370877, 472117

Review URL: https://codereview.chromium.org/1064823002

Cr-Commit-Position: refs/heads/master@{#324430}