Add debug method to print surface references.

cc/surfaces/surface.h

Index: cc/surfaces/surface.h
diff --git a/cc/surfaces/surface.h b/cc/surfaces/surface.h
index cea948fb2f64af75982a984f892bc6cc5d12c2b4..0ac97a13b05bd675b9b941daceea0c8e7be6f050 100644
--- a/cc/surfaces/surface.h
+++ b/cc/surfaces/surface.h
@@ -61,7 +61,7 @@ class CC_SURFACES_EXPORT Surface {
 
   // 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_; }