Allow top_sites to be null.

iOS wants it for tests. Everywhere we use top_sites_, we check for its
non-null-ness.

BUG=631990

Review-Url: https://codereview.chromium.org/2395483002
Cr-Commit-Position: refs/heads/master@{#422821}

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);

components/ntp_tiles/most_visited_sites.h

@@ -113,8 +113,8 @@ class MostVisitedSites : public history::TopSitesObserver,
 
   // Construct a MostVisitedSites instance.
   //
-  // |prefs|, |top_sites|, and |suggestions| are required and may not be null.
-  // |popular_sites| and |supervisor| are optional and if null the associated
+  // |prefs| and |suggestions| are required and may not be null. |top_sites|,
+  // |popular_sites|, and |supervisor| are optional and if null the associated
   // features will be disabled.
   MostVisitedSites(PrefService* prefs,
                    scoped_refptr<history::TopSites> top_sites,