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 df334aa363fcf008e07ce0879822b8fcc66f3265..e4c65c886e9ff0a23bed89dd09ca8673af634597 100644
--- a/cc/layers/picture_layer_impl.cc
+++ b/cc/layers/picture_layer_impl.cc
@@ -375,6 +375,10 @@ void PictureLayerImpl::DidBecomeActive() {
   tilings_->DidBecomeActive();
 }
 
+void PictureLayerImpl::DidBeginTracing() {
+  pile_->DidBeginTracing();
+}
+
 void PictureLayerImpl::DidLoseOutputSurface() {
   if (tilings_)
     tilings_->RemoveAllTilings();