Allow top_sites to be null.

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 6917269cd739221b6e28799664b614187ea001db..309a0b48d1568e234d25a7e34c950e3634eaf245 100644
--- a/components/ntp_tiles/most_visited_sites.cc
+++ b/components/ntp_tiles/most_visited_sites.cc
@@ -159,7 +159,8 @@ MostVisitedSites::MostVisitedSites(PrefService* prefs,
       mv_source_(NTPTileSource::SUGGESTIONS_SERVICE),
       weak_ptr_factory_(this) {
   DCHECK(prefs_);
-  DCHECK(top_sites_);
+  // top_sites_ can be null in tests.
+  // TODO(sfiera): have iOS use a dummy TopSites in its tests.
   DCHECK(suggestions_service_);
   if (supervisor_)
     supervisor_->SetObserver(this);