This is a reland of d63652c8

The new version reverts a change in tests (removal of
urls_with_favicons_) which presumably introduces flakiness on Mac.
My hypothesis is that using urls_with_favicons_ hides a race
condition that predates my patch, because the failing test is
unrelated to favicons, TwoClientBookmarksSyncTest.Sanity, and the
failing logs surface title mismatches as well:
[...:bookmarks_helper.cc(377)] Title mismatch: Yahoo vs. Google

Original change's description:
> Extend bookmark sync code with support for favicon deletion
> 
> Recent patches have introduced new favicon scenarios by handling cases
> where a site's content changes to not contain favicons.
> 
> Updating sync code to handle these events is needed to prevent sync
> from resurrecting the favicon, due to not knowing about the deletion
> and hence sync-ing it back to the local cache.
> 
> One notable change is that the new logic will introduce the concept of
> sync-ed bookmark protos having neither an icon URL (field |icon_url|)
> nor favicon image data (field |favicon|). Ancient versions of chrome
> (prior to M25) didn't sync these fields and would be incompatible
> (although not really badly) but too old for us to care.
> 
> Manually tested sync-ing across two patched instances of Chrome desktop
> (Linux):
> - Propagate deletions of bookmark favicons (after page content changes).
> - Restoring deleted bookmark favicons (after page content changes).
> 
> Bug: 85806
> Change-Id: Ia3534c6094983658f59f973777b5ed649c8769e8
> Reviewed-on: https://chromium-review.googlesource.com/704737


> Commit-Queue: Mikel Astiz <mastiz@chromium.org>
> Reviewed-by: Scott Violet <sky@chromium.org>
> Reviewed-by: Nicolas Zea <zea@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#514424}

Bug: 85806
Change-Id: If4d05da9e13c1d9d5abb19fd8a7c1d463af609a4
Reviewed-on: https://chromium-review.googlesource.com/757516


Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Nicolas Zea <zea@chromium.org>
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#514934}