TopSites: Clear thumbnails from the cache when their URLs get removed

We already cleared the thumbnails from persistent storage, but they
remained in the in-memory cache, so they remained accessible (until the
next Chrome restart) even after all browsing data was cleared.

Bug: 758169
Change-Id: Id916d22358430a82e6d5043ac04fa463a32f824f
Reviewed-on: https://chromium-review.googlesource.com/758640
Commit-Queue: Marc Treib <treib@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#514861}
bool TopSitesImpl::IsKnownURL(const GURL& url) {
  return loaded_ && cache_->IsKnownURL(url);
}

bool TopSitesImpl::IsKnownURL(const GURL& url) {
  return loaded_ && cache_->IsKnownURL(url);
}
