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This moves all the favicon related functionality out of
HistoryBackend into FaviconBackend. FaviconBackend is owned
by HistoryBackend. FaviconBackend indirectly talks to
HistoryBackend by way FaviconBackendDelegate.

BUG=1076463
TEST=covered by tests

Change-Id: If9927019b4539174d36daaf02597fcf1f1ad5c35
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2314990Reviewed-by: Peter Kotwicz <pkotwicz@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#792007}