Add a DCHECK to RenderWidgetHost::FromID to ensure that it's only called on the UI thread.

R=avi@chromium.org

Review URL: https://codereview.chromium.org/23471009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@221750 0039d316-1c4b-4281-b951-d872f2087c98

content/browser/renderer_host/render_widget_host_impl.cc

@@ -226,6 +226,7 @@ RenderWidgetHost* RenderWidgetHost::FromID(
 RenderWidgetHostImpl* RenderWidgetHostImpl::FromID(
     int32 process_id,
     int32 routing_id) {
+  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
   RoutingIDWidgetMap* widgets = g_routing_id_widget_map.Pointer();
   RoutingIDWidgetMap::iterator it = widgets->find(
       RenderWidgetHostID(process_id, routing_id));