Keep track on the layer tree construction time in the animator. (#2968)

flow/layers/layer_tree.h

@@ -20,6 +20,7 @@ namespace flow {
 class LayerTree {
  public:
   LayerTree();
+
   ~LayerTree();
 
   void Raster(CompositorContext::ScopedFrame& frame);
@@ -41,6 +42,7 @@ class LayerTree {
   void set_frame_size(const SkISize& frame_size) { frame_size_ = frame_size; }
 
   uint32_t scene_version() const { return scene_version_; }
+
   void set_scene_version(uint32_t scene_version) {
     scene_version_ = scene_version;
   }

sky/shell/ui/animator.cc

@@ -64,10 +64,17 @@ void Animator::BeginFrame(int64_t time_stamp) {
   // to service potential frame.
   DCHECK(producer_continuation_);
 
-  engine_->BeginFrame(ftl::TimePoint::Now());
+  last_begin_frame_time_ = ftl::TimePoint::Now();
+  engine_->BeginFrame(last_begin_frame_time_);
 }
 
 void Animator::Render(std::unique_ptr<flow::LayerTree> layer_tree) {
+  if (layer_tree) {
+    // Note the frame time for instrumentation.
+    layer_tree->set_construction_time(ftl::TimePoint::Now() -
+                                      last_begin_frame_time_);
+  }
+
   // Commit the pending continuation.
   producer_continuation_.Complete(std::move(layer_tree));
 

sky/shell/ui/animator.h

@@ -45,6 +45,7 @@ class Animator {
   Engine* engine_;
   vsync::VSyncProviderPtr vsync_provider_;
   vsync::VSyncProviderPtr fallback_vsync_provider_;
+  ftl::TimePoint last_begin_frame_time_;
   ftl::RefPtr<LayerTreePipeline> layer_tree_pipeline_;
   flutter::Semaphore pending_frame_semaphore_;
   LayerTreePipeline::ProducerContinuation producer_continuation_;