Chromecast: use CastContent{Browser,Renderer}Client as a base class.

chromecast/renderer/cast_content_renderer_client.h

Index: chromecast/renderer/cast_content_renderer_client.h
diff --git a/chromecast/renderer/cast_content_renderer_client.h b/chromecast/renderer/cast_content_renderer_client.h
index 41390ac23bff477a53bc395db43cb5f799f8a340..f8539f6102efe57ba8951355b16b458022bf8159 100644
--- a/chromecast/renderer/cast_content_renderer_client.h
+++ b/chromecast/renderer/cast_content_renderer_client.h
@@ -27,7 +27,10 @@ void PlatformAddRendererNativeBindings(blink::WebLocalFrame* frame);
 
 class CastContentRendererClient : public content::ContentRendererClient {
  public:
-  CastContentRendererClient();
+  // Creates an implementation of CastContentRendererClient. Platform should
+  // link in an implementation as needed.
+  static scoped_ptr<CastContentRendererClient> Create();
+
   ~CastContentRendererClient() override;
 
   // Returns any MessageFilters from the platform implementation that should
@@ -49,6 +52,9 @@ class CastContentRendererClient : public content::ContentRendererClient {
   void DeferMediaLoad(content::RenderFrame* render_frame,
                       const base::Closure& closure) override;
 
+ protected:
+  CastContentRendererClient();
+
  private:
   scoped_ptr<network_hints::PrescientNetworkingDispatcher>
       prescient_networking_dispatcher_;