cc: Report only on active tiles in tracing.

cc/layers/picture_layer_impl.cc

Index: cc/layers/picture_layer_impl.cc
diff --git a/cc/layers/picture_layer_impl.cc b/cc/layers/picture_layer_impl.cc
index ede7da0ac3a5853216a7103b38852af889669b7e..bf65940fe033d74181ab31629af9d61c8f8a9860 100644
--- a/cc/layers/picture_layer_impl.cc
+++ b/cc/layers/picture_layer_impl.cc
@@ -6,6 +6,7 @@
 
 #include <algorithm>
 #include <limits>
+#include <set>
 
 #include "base/debug/trace_event_argument.h"
 #include "base/time/time.h"
@@ -1337,6 +1338,15 @@ void PictureLayerImpl::GetDebugBorderProperties(
   *width = DebugColors::TiledContentLayerBorderWidth(layer_tree_impl());
 }
 
+void PictureLayerImpl::AllTilesForTracingInto(
+    std::set<const Tile*>* tiles) const {
+  if (!tilings_)
+    return;
+
+  for (size_t i = 0; i < tilings_->num_tilings(); ++i)
+    tilings_->tiling_at(i)->AllTilesForTracingInto(tiles);
+}
+
 void PictureLayerImpl::AsValueInto(base::debug::TracedValue* state) const {
   const_cast<PictureLayerImpl*>(this)->DoPostCommitInitializationIfNeeded();
   LayerImpl::AsValueInto(state);