Add TRACE_EVENT_IS_NEW_TRACE as a way to snapshot objects at start of recording

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 72a878b9de98ec2b4a201e871039dd33357d187d..6c77c2205c3877dfaab3822a29b9781c06148130 100644
--- a/cc/layers/picture_layer_impl.cc
+++ b/cc/layers/picture_layer_impl.cc
@@ -376,6 +376,10 @@ void PictureLayerImpl::DidBecomeActive() {
   tilings_->DidBecomeActive();
 }
 
+void PictureLayerImpl::DidBeginTracing() {
+  pile_->DidBeginTracing();
+}
+
 void PictureLayerImpl::DidLoseOutputSurface() {
   if (tilings_)
     tilings_->RemoveAllTilings();