Add source frame number to LayerTreeImpl::AsValueInto

Will help work around https://code.google.com/p/trace-viewer/issues/detail?id=587

R=enne

Review URL: https://codereview.chromium.org/495423003

Cr-Commit-Position: refs/heads/master@{#291573}

cc/trees/layer_tree_impl.cc

@@ -819,6 +819,7 @@ AnimationRegistrar* LayerTreeImpl::animationRegistrar() const {
 
 void LayerTreeImpl::AsValueInto(base::debug::TracedValue* state) const {
   TracedValue::MakeDictIntoImplicitSnapshot(state, "cc::LayerTreeImpl", this);
+  state->SetInteger("source_frame_number", source_frame_number_);
 
   state->BeginDictionary("root_layer");
   root_layer_->AsValueInto(state);