cc: Report only on active tiles in tracing.

cc/resources/picture_layer_tiling.cc

Index: cc/resources/picture_layer_tiling.cc
diff --git a/cc/resources/picture_layer_tiling.cc b/cc/resources/picture_layer_tiling.cc
index 947bb21b6b84a2de39bd75cc0717c63dec49cee9..5512d530f4fab31fa9ccd163c583c67481232694 100644
--- a/cc/resources/picture_layer_tiling.cc
+++ b/cc/resources/picture_layer_tiling.cc
@@ -7,6 +7,7 @@
 #include <algorithm>
 #include <cmath>
 #include <limits>
+#include <set>
 
 #include "base/debug/trace_event.h"
 #include "base/debug/trace_event_argument.h"
@@ -668,6 +669,12 @@ void PictureLayerTiling::DidBecomeActive() {
   }
 }
 
+void PictureLayerTiling::AllTilesForTracingInto(
+    std::set<const Tile*>* tiles) const {
+  for (TileMap::const_iterator it = tiles_.begin(); it != tiles_.end(); ++it)
+    tiles->insert(it->second.get());
+}
+
 void PictureLayerTiling::AsValueInto(base::debug::TracedValue* state) const {
   state->SetInteger("num_tiles", tiles_.size());
   state->SetDouble("content_scale", contents_scale_);