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}