Explicitly call OnModelChanged when there are no titles to load

Fixs several release blocking regressions in the behaviour of the
startup page list. Lamentably there are no tests for this code and
adding new ones looks tricky (we'd either need to fix crbug.com/419370)
or refactor the classes to be testable.

BUG=452370,452407

Review URL: https://codereview.chromium.org/879953002

Cr-Commit-Position: refs/heads/master@{#313487}

chrome/browser/custom_home_pages_table_model.cc

@@ -260,6 +260,8 @@ void CustomHomePagesTableModel::LoadAllTitles() {
           &task_tracker_);
     }
   }
+  if (entries_.empty())
+    observer_->OnModelChanged();
 }
 
 void CustomHomePagesTableModel::OnGotOneOfManyTitles(const GURL& entry_url,