cc: Don't do CompositeAndReadback when using DelegatingRenderer.

cc/output/renderer.h

 // Copyright 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.
 
 #ifndef CC_OUTPUT_RENDERER_H_
 #define CC_OUTPUT_RENDERER_H_
 
 #include "base/basictypes.h"
 #include "cc/base/cc_export.h"
 #include "cc/debug/latency_info.h"
@@ skipping 32 matching lines @@
 
   const LayerTreeSettings& Settings() const { return client_->Settings(); }
 
   gfx::Size ViewportSize() const { return client_->DeviceViewportSize(); }
   int ViewportWidth() const { return ViewportSize().width(); }
   int ViewportHeight() const { return ViewportSize().height(); }
 
   virtual void ViewportChanged() {}
   virtual void ReceiveCompositorFrameAck(const CompositorFrameAck& ack) {}
 
+  virtual bool CanReadPixels() const = 0;
+
   virtual void DecideRenderPassAllocationsForFrame(
       const RenderPassList& render_passes_in_draw_order) {}
   virtual bool HaveCachedResourcesForRenderPassId(RenderPass::Id id) const;
 
   // This passes ownership of the render passes to the renderer. It should
   // consume them, and empty the list.
   virtual void DrawFrame(RenderPassList* render_passes_in_draw_order) = 0;
 
   // Waits for rendering to finish.
   virtual void Finish() = 0;
@@ skipping 18 matching lines @@
       : client_(client) {}
 
   RendererClient* client_;
 
   DISALLOW_COPY_AND_ASSIGN(Renderer);
 };
 
 }  // namespace cc
 
 #endif  // CC_OUTPUT_RENDERER_H_