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

content/browser/renderer_host/render_widget_host_impl.cc

 // Copyright (c) 2012 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 "content/browser/renderer_host/render_widget_host_impl.h"
 
 #include <math.h>
 #include <utility>
 
 #include "base/auto_reset.h"
@@ skipping 208 matching lines @@
 RenderWidgetHost* RenderWidgetHost::FromID(
     int32 process_id,
     int32 routing_id) {
   return RenderWidgetHostImpl::FromID(process_id, routing_id);
 }
 
 // static
 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));
   return it == widgets->end() ? NULL : it->second;
 }
 
 // static
 std::vector<RenderWidgetHost*> RenderWidgetHost::GetRenderWidgetHosts() {
   std::vector<RenderWidgetHost*> hosts;
   RoutingIDWidgetMap* widgets = g_routing_id_widget_map.Pointer();
@@ skipping 2271 matching lines @@
     int process_id = (b->first.second >> 32) & 0xffffffff;
     RenderWidgetHost* rwh =
         RenderWidgetHost::FromID(process_id, routing_id);
     if (!rwh)
       continue;
     RenderWidgetHostImpl::From(rwh)->FrameSwapped(latency_info);
   }
 }
 
 }  // namespace content