cc: Add UMA to measure the raster duration for pending tree activation.

cc/trees/layer_tree_host_impl.h

Index: cc/trees/layer_tree_host_impl.h
diff --git a/cc/trees/layer_tree_host_impl.h b/cc/trees/layer_tree_host_impl.h
index 611f24b77fc3db578a919622ffb3643d02f6207d..b4007e001b12aa1a68c2e544eb0c1d3422ec05d8 100644
--- a/cc/trees/layer_tree_host_impl.h
+++ b/cc/trees/layer_tree_host_impl.h
@@ -66,6 +66,7 @@ class MutatorEvents;
 class MutatorHost;
 class PageScaleAnimation;
 class PendingTreeDurationHistogramTimer;
+class PendingTreeRasterDurationHistogramTimer;
 class RasterTilePriorityQueue;
 class RasterBufferProvider;
 class RenderingStatsInstrumentation;
@@ -861,6 +862,8 @@ class CC_EXPORT LayerTreeHostImpl
 
   std::unique_ptr<PendingTreeDurationHistogramTimer>
       pending_tree_duration_timer_;
+  std::unique_ptr<PendingTreeRasterDurationHistogramTimer>
+      pending_tree_raster_duration_timer_;
 
   // The id of the scroll node to which scroll animations must latch.
   // This gets reset at ScrollAnimatedBegin, and updated the first time that a