cc: Report only on active tiles in tracing.

cc/resources/picture_layer_tiling.h

Index: cc/resources/picture_layer_tiling.h
diff --git a/cc/resources/picture_layer_tiling.h b/cc/resources/picture_layer_tiling.h
index 415ad4279e0b543e624332522196e2ebb9e60bc2..e859146f0a48f7a94bded23037349ee3c8291cf4 100644
--- a/cc/resources/picture_layer_tiling.h
+++ b/cc/resources/picture_layer_tiling.h
@@ -5,6 +5,7 @@
 #ifndef CC_RESOURCES_PICTURE_LAYER_TILING_H_
 #define CC_RESOURCES_PICTURE_LAYER_TILING_H_
 
+#include <set>
 #include <utility>
 #include <vector>
 
@@ -259,6 +260,7 @@ class CC_EXPORT PictureLayerTiling {
     return frame_time_in_seconds != last_impl_frame_time_in_seconds_;
   }
 
+  void AllTilesForTracingInto(std::set<const Tile*>* tiles) const;
   void AsValueInto(base::debug::TracedValue* array) const;
   size_t GPUMemoryUsageInBytes() const;