blimp: Use remote client LayerTreeHost in the BlimpCompositor.

blimp/client/feature/render_widget_feature.h

Index: blimp/client/feature/render_widget_feature.h
diff --git a/blimp/client/feature/render_widget_feature.h b/blimp/client/feature/render_widget_feature.h
index cb938d740d4553cd8a95b7eae4b26e7b91ec2ee5..14a9f26734f2090ea36ea53613014a70bd3ff00b 100644
--- a/blimp/client/feature/render_widget_feature.h
+++ b/blimp/client/feature/render_widget_feature.h
@@ -74,8 +74,9 @@ class BLIMP_CLIENT_EXPORT RenderWidgetFeature : public BlimpMessageProcessor {
   // Sets a RenderWidgetMessageDelegate to be notified of all incoming
   // RenderWidget related messages for |tab_id| from the engine.  There can only
   // be one RenderWidgetMessageDelegate per tab.
-  void SetDelegate(const int tab_id, RenderWidgetFeatureDelegate* delegate);
-  void RemoveDelegate(const int tab_id);
+  // virtual for testing.
+  virtual void SetDelegate(int tab_id, RenderWidgetFeatureDelegate* delegate);
+  virtual void RemoveDelegate(const int tab_id);
 
  private:
   // BlimpMessageProcessor implementation.