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

cc/trees/layer_tree_host_impl.cc

Index: cc/trees/layer_tree_host_impl.cc
diff --git a/cc/trees/layer_tree_host_impl.cc b/cc/trees/layer_tree_host_impl.cc
index 84c43c20b1c3462b342036d0fbbc7311e9249e28..678440e31506440f8a67fefd218a097c99a1b16d 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -1096,6 +1096,13 @@ bool LayerTreeHostImpl::AllowPartialSwap() const {
   return !debug_state_.ShowHudRects();
 }
 
+class DidBeginTracingFunctor {
+ public:
+  void operator()(LayerImpl* layer) {
+    layer->DidBeginTracing();
+  }
+};
+
 void LayerTreeHostImpl::DrawLayers(FrameData* frame,
                                    base::TimeTicks frame_begin_time) {
   TRACE_EVENT0("cc", "LayerTreeHostImpl::DrawLayers");
@@ -1133,6 +1140,18 @@ void LayerTreeHostImpl::DrawLayers(FrameData* frame,
     paint_time_counter_->SavePaintTime(stats.total_paint_time);
   }
 
+  bool is_new_trace;
+  TRACE_EVENT_IS_NEW_TRACE(&is_new_trace);
+  if (is_new_trace) {
+    if (pending_tree_)
+      LayerTreeHostCommon::CallFunctionForSubtree<
+          DidBeginTracingFunctor, LayerImpl>(
+              pending_tree_->root_layer());
+    LayerTreeHostCommon::CallFunctionForSubtree<
+        DidBeginTracingFunctor, LayerImpl>(
+            active_tree_->root_layer());
+  }
+
   TRACE_EVENT_OBJECT_SNAPSHOT_WITH_ID(
       TRACE_DISABLED_BY_DEFAULT("cc.debug"), "cc::LayerTreeHostImpl", this,
       TracedValue::FromValue(AsValue().release()));