Add an uma stat to time Pending Tree activation

cc/trees/layer_tree_host_impl.h

 // 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.
 
 #ifndef CC_TREES_LAYER_TREE_HOST_IMPL_H_
 #define CC_TREES_LAYER_TREE_HOST_IMPL_H_
 
 #include <stddef.h>
 
 #include <bitset>
@@ skipping 43 matching lines @@
 class AnimationHost;
 class CompletionEvent;
 class CompositorFrameMetadata;
 class DebugRectHistory;
 class EvictionTilePriorityQueue;
 class FrameRateCounter;
 class LayerImpl;
 class LayerTreeImpl;
 class MemoryHistory;
 class PageScaleAnimation;
+class PendingTreeDurationHistogramTimer;
 class PictureLayerImpl;
 class RasterTilePriorityQueue;
 class TileTaskManager;
 class RasterBufferProvider;
 class RenderPassDrawQuad;
 class RenderingStatsInstrumentation;
 class ResourcePool;
 class ScrollElasticityHelper;
 class ScrollbarLayerImplBase;
 class SwapPromise;
@@ skipping 771 matching lines @@
   bool requires_high_res_to_draw_;
   bool is_likely_to_require_a_draw_;
 
   std::unique_ptr<Viewport> viewport_;
 
   LayerTreeMutator* mutator_;
 
   bool has_fixed_raster_scale_blurry_content_;
   std::bitset<kFixedRasterScaleAttemptedScaleChangeHistoryCount>
       fixed_raster_scale_attempted_scale_change_history_;
+  std::unique_ptr<PendingTreeDurationHistogramTimer>
+      pending_tree_duration_timer_;
 
   DISALLOW_COPY_AND_ASSIGN(LayerTreeHostImpl);
 };
 
 }  // namespace cc
 
 #endif  // CC_TREES_LAYER_TREE_HOST_IMPL_H_