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 b3a095f5c5a01d0a408707fca288f15534937fe1..43348fa5000e31ca26b34a48665d1d26598b47c6 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"
@@ -1404,6 +1405,15 @@ void PictureLayerImpl::GetDebugBorderProperties(
   *width = DebugColors::TiledContentLayerBorderWidth(layer_tree_impl());
 }
 
+void PictureLayerImpl::GetAllTilesForTracing(
+    std::set<const Tile*>* tiles) const {
+  if (!tilings_)
+    return;
+
+  for (size_t i = 0; i < tilings_->num_tilings(); ++i)
+    tilings_->tiling_at(i)->GetAllTilesForTracing(tiles);
+}
+
 void PictureLayerImpl::AsValueInto(base::debug::TracedValue* state) const {
   const_cast<PictureLayerImpl*>(this)->DoPostCommitInitializationIfNeeded();
   LayerImpl::AsValueInto(state);