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}
void AppendMostVisitedURLwithTitle(const GURL& url,
                                   const base::string16& title,
                                   std::vector<MostVisitedURL>* list) {
  MostVisitedURL mv;
  mv.url = url;
  mv.title = title;
  mv.redirects.push_back(url);
  list->push_back(mv);
}

void AppendMostVisitedURLwithTitle(const GURL& url,
                                   const base::string16& title,
                                   std::vector<MostVisitedURL>* list) {
  MostVisitedURL mv;
  mv.url = url;
  mv.title = title;
  mv.redirects.push_back(url);
  list->push_back(mv);
}
