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

cc/trees/layer_tree_host_impl.cc

 // Copyright 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/trees/layer_tree_host_impl.h"
 
 #include <algorithm>
 
 #include "base/basictypes.h"
 #include "base/json/json_writer.h"
@@ skipping 1078 matching lines @@
   return metadata;
 }
 
 bool LayerTreeHostImpl::AllowPartialSwap() const {
   // We don't track damage on the HUD layer (it interacts with damage tracking
   // visualizations), so disable partial swaps to make the HUD layer display
   // properly.
   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");
   DCHECK(CanDraw());
 
   if (frame->has_no_damage)
     return;
 
   DCHECK(!frame->render_passes.empty());
 
@@ skipping 17 matching lines @@
         frame->non_occluding_screen_space_rects,
         debug_state_);
   }
 
   if (!settings_.impl_side_painting && debug_state_.continuous_painting) {
     const RenderingStats& stats =
         rendering_stats_instrumentation_->GetRenderingStats();
     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()));
 
   // Because the contents of the HUD depend on everything else in the frame, the
   // contents of its texture are updated as the last thing before the frame is
   // drawn.
   if (active_tree_->hud_layer())
     active_tree_->hud_layer()->UpdateHudTexture(resource_provider_.get());
 
@@ skipping 1096 matching lines @@
 }
 
 void LayerTreeHostImpl::SetDebugState(const LayerTreeDebugState& debug_state) {
   if (debug_state_.continuous_painting != debug_state.continuous_painting)
     paint_time_counter_->ClearHistory();
 
   debug_state_ = debug_state;
 }
 
 }  // namespace cc