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Previously, MostVisitedSites dumped all PopularSites information into
its own observer, then let Java ensure availability of icons. Popular
Sites on iOS needs these icons cached too, so the component should
handle icon availability itself.

MostVisitedSites::Observer::OnPopularURLsAvailable() is removed and
MostVisitedSites::Observer::OnIconMadeAvailable() is added in its place.

Note that FaviconHelper.ensureIconIsAvailable() hasn't been removed in
this CL, as it's still used to load favicons for snippets. Of course,
one day, iOS will need to do that too, so an eventual plan to share
IconCacher and broaden its interface makes sense (maybe coupled with a
move to the favicon component?).

BUG=631990

Review-Url: https://codereview.chromium.org/2388783004
Cr-Commit-Position: refs/heads/master@{#427100}