Use a struct for cc::Surface ids for more type safety

cc/surfaces/display.h

Index: cc/surfaces/display.h
diff --git a/cc/surfaces/display.h b/cc/surfaces/display.h
index 1f8bf1756e77a44467c186ca057a691be684e5cc..9c401cfd6e6d83ec1850458826538d18717dd7dd 100644
--- a/cc/surfaces/display.h
+++ b/cc/surfaces/display.h
@@ -11,6 +11,7 @@
 #include "cc/output/renderer.h"
 #include "cc/surfaces/surface_aggregator.h"
 #include "cc/surfaces/surface_client.h"
+#include "cc/surfaces/surface_id.h"
 #include "cc/surfaces/surfaces_export.h"
 
 namespace gfx {
@@ -39,7 +40,7 @@ class CC_SURFACES_EXPORT Display : public SurfaceClient,
   void Resize(const gfx::Size& new_size);
   bool Draw();
 
-  int CurrentSurfaceID();
+  SurfaceId CurrentSurfaceId();
 
   // OutputSurfaceClient implementation.
   virtual void DeferredInitialize() OVERRIDE {}