Add an uma stat to time Pending Tree activation

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 <stddef.h>
 #include <stdint.h>
 
 #include <algorithm>
@@ skipping 146 matching lines @@
     UMA_HISTOGRAM_BOOLEAN("Renderer4.CompositorWheelScrollUpdateThread",
                           scroll_on_main_thread);
   } else {
     UMA_HISTOGRAM_BOOLEAN("Renderer4.CompositorTouchScrollUpdateThread",
                           scroll_on_main_thread);
   }
 }
 
 }  // namespace
 
+DEFINE_SCOPED_UMA_HISTOGRAM_TIMER(PendingTreeDurationHistogramTimer,
+                                  "Scheduling.%s.PendingTreeDuration");
+
 LayerTreeHostImpl::FrameData::FrameData()
     : render_surface_layer_list(nullptr), has_no_damage(false) {}
 
 LayerTreeHostImpl::FrameData::~FrameData() {}
 
 std::unique_ptr<LayerTreeHostImpl> LayerTreeHostImpl::Create(
     const LayerTreeSettings& settings,
     LayerTreeHostImplClient* client,
     TaskRunnerProvider* task_runner_provider,
     RenderingStatsInstrumentation* rendering_stats_instrumentation,
@@ skipping 1776 matching lines @@
   if (recycle_tree_)
     recycle_tree_.swap(pending_tree_);
   else
     pending_tree_ =
         LayerTreeImpl::create(this, active_tree()->page_scale_factor(),
                               active_tree()->top_controls_shown_ratio(),
                               active_tree()->elastic_overscroll());
 
   client_->OnCanDrawStateChanged(CanDraw());
   TRACE_EVENT_ASYNC_BEGIN0("cc", "PendingTree:waiting", pending_tree_.get());
+
+  DCHECK(!pending_tree_duration_timer_);
+  pending_tree_duration_timer_.reset(new PendingTreeDurationHistogramTimer());
 }
 
 void LayerTreeHostImpl::ActivateSyncTree() {
   if (pending_tree_) {
     TRACE_EVENT_ASYNC_END0("cc", "PendingTree:waiting", pending_tree_.get());
 
+    DCHECK(pending_tree_duration_timer_);
+    // Reset will call the destructor and log the timer histogram.
+    pending_tree_duration_timer_.reset();
+
     // Process any requests in the UI resource queue.  The request queue is
     // given in LayerTreeHost::FinishCommitOnImplThread.  This must take place
     // before the swap.
     pending_tree_->ProcessUIResourceRequestQueue();
 
     if (pending_tree_->needs_full_tree_sync()) {
       TreeSynchronizer::SynchronizeTrees(pending_tree_->root_layer(),
                                          active_tree_.get());
     }
 
@@ skipping 2024 matching lines @@
     const {
   return fixed_raster_scale_attempted_scale_change_history_.count() >=
          kFixedRasterScaleAttemptedScaleChangeThreshold;
 }
 
 void LayerTreeHostImpl::SetFixedRasterScaleAttemptedToChangeScale() {
   fixed_raster_scale_attempted_scale_change_history_.set(0);
 }
 
 }  // namespace cc