Allow top_sites to be null.

components/ntp_tiles/most_visited_sites.cc

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "components/ntp_tiles/most_visited_sites.h"
 
 #include <algorithm>
 #include <set>
 #include <string>
 #include <utility>
@@ skipping 141 matching lines @@
       supervisor_(supervisor),
       observer_(nullptr),
       num_sites_(0),
       waiting_for_most_visited_sites_(true),
       waiting_for_popular_sites_(true),
       recorded_uma_(false),
       top_sites_observer_(this),
       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);
 }
 
 MostVisitedSites::~MostVisitedSites() {
   if (supervisor_)
     supervisor_->SetObserver(nullptr);
 }
 
@@ skipping 371 matching lines @@
 
 void MostVisitedSites::TopSitesChanged(TopSites* top_sites,
                                        ChangeReason change_reason) {
   if (mv_source_ == NTPTileSource::TOP_SITES) {
     // The displayed tiles are invalidated.
     InitiateTopSitesQuery();
   }
 }
 
 }  // namespace ntp_tiles