Add debug method to print surface references.

cc/surfaces/surface.h

 // Copyright 2014 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_SURFACES_SURFACE_H_
 #define CC_SURFACES_SURFACE_H_
 
 #include <stddef.h>
 #include <stdint.h>
 
@@ skipping 43 matching lines @@
   void QueueFrame(CompositorFrame frame, const DrawCallback& draw_callback);
   void EvictFrame();
   void RequestCopyOfOutput(std::unique_ptr<CopyOutputRequest> copy_request);
   // Adds each CopyOutputRequest in the current frame to copy_requests. The
   // caller takes ownership of them. |copy_requests| is keyed by RenderPass ids.
   void TakeCopyOutputRequests(
       std::multimap<int, std::unique_ptr<CopyOutputRequest>>* copy_requests);
 
   // Returns the most recent frame that is eligible to be rendered.
   // You must check whether HasFrame() returns true before calling this method.
-  const CompositorFrame& GetEligibleFrame();
+  const CompositorFrame& GetEligibleFrame() const;
 
   // Returns a number that increments by 1 every time a new frame is enqueued.
   int frame_index() const { return frame_index_; }
 
   void TakeLatencyInfo(std::vector<ui::LatencyInfo>* latency_info);
   void RunDrawCallbacks();
 
   base::WeakPtr<SurfaceFactory> factory() { return factory_; }
 
   // Add a SurfaceSequence that must be satisfied before the Surface is
@@ skipping 38 matching lines @@
   std::vector<SurfaceId> referenced_surfaces_;
 
   DrawCallback draw_callback_;
 
   DISALLOW_COPY_AND_ASSIGN(Surface);
 };
 
 }  // namespace cc
 
 #endif  // CC_SURFACES_SURFACE_H_