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This CL prevents sync from updating the "last updated time" for bookmark
favicons each time that Chrome is started. The "last updated time" determines
when the favicon is redownloaded from the web (in case it has been updated).

Sync calls HistoryBackend::MergeFavicon() for each bookmark favicon on startup.
This CL stops HistoryBackend::MergeFavicon() from updating the "last updated
time" when MergeFavicon() is called with the same bitmap data as already stored
in the database (no new bitmap data).

BUG=481414
TEST=TwoClientBookmarksSyncTest.SC_UpdatingTitleDoesNotUpdateFaviconLastUpdatedTime

Review URL: https://codereview.chromium.org/1128343004

Cr-Commit-Position: refs/heads/master@{#330802}