[Popular Sites] Fix loading latency noticeable on iOS (again)
Cached Popular Sites are now read synchronously (via tiles()) instead
of waiting for the callback. The callback is expected to be almost
immediate, but it turns out that the latency is easily noticeable on
iOS.
Since we have cached and parsed JSON, the list of sites can be built
before returning from StartFetch (now renamed to MaybeStartFetch())
and hence MostVisitedSites is able to expose PopularSites in its
first notification to observers.
The fix has been verified on an iOS device.
BUG=662397
Review-Url: https://codereview.chromium.org/2659633002
Cr-Commit-Position: refs/heads/master@{#446736}
9 files changed
