ntp_tiles: Add tests covering tile-fetching logic

components/ntp_tiles/most_visited_sites.cc

Index: components/ntp_tiles/most_visited_sites.cc
diff --git a/components/ntp_tiles/most_visited_sites.cc b/components/ntp_tiles/most_visited_sites.cc
index 35bfd4472236aa1aba96a84fc47eae3af3d16546..376c4e45a4fe181c474c30c9e1422174c8f07eff 100644
--- a/components/ntp_tiles/most_visited_sites.cc
+++ b/components/ntp_tiles/most_visited_sites.cc
@@ -187,6 +187,8 @@ base::FilePath MostVisitedSites::GetWhitelistLargeIconPath(const GURL& url) {
 
 void MostVisitedSites::OnMostVisitedURLsAvailable(
     const history::MostVisitedURLList& visited_list) {
+  // TODO(mastiz): Verify if suggestion service results have been fetched in
+  // the meantime, and if that's the case ignore this event.
   NTPTilesVector tiles;
   size_t num_tiles =
       std::min(visited_list.size(), static_cast<size_t>(num_sites_));