cc : Add UMA metric for finding closest matching layer to a point

cc/trees/layer_tree_impl.cc

Index: cc/trees/layer_tree_impl.cc
diff --git a/cc/trees/layer_tree_impl.cc b/cc/trees/layer_tree_impl.cc
index 1cf9fc54829a1078171a800e162db971d5be9cdc..5cb6ddb43da2608cbcf4c1696e5d529f1caff3e0 100644
--- a/cc/trees/layer_tree_impl.cc
+++ b/cc/trees/layer_tree_impl.cc
@@ -1881,32 +1881,37 @@ static void FindClosestMatchingLayer(const gfx::PointF& screen_space_point,
                                      const Functor& func,
                                      FindClosestMatchingLayerState* state) {
   // We want to iterate from front to back when hit testing.
-  for (auto* layer : base::Reversed(*root_layer->layer_tree_impl())) {
-    if (!func(layer))
-      continue;
+  {
+    base::ElapsedTimer timer;
+    for (auto* layer : base::Reversed(*root_layer->layer_tree_impl())) {
+      if (!func(layer))
+        continue;
 
-    float distance_to_intersection = 0.f;
-    bool hit = false;
-    if (layer->Is3dSorted())
-      hit =
-          PointHitsLayer(layer, screen_space_point, &distance_to_intersection);
-    else
-      hit = PointHitsLayer(layer, screen_space_point, nullptr);
+      float distance_to_intersection = 0.f;
+      bool hit = false;
+      if (layer->Is3dSorted())
+        hit = PointHitsLayer(layer, screen_space_point,
+                             &distance_to_intersection);
+      else
+        hit = PointHitsLayer(layer, screen_space_point, nullptr);
 
-    if (!hit)
-      continue;
+      if (!hit)
+        continue;
 
-    bool in_front_of_previous_candidate =
-        state->closest_match &&
-        layer->GetSortingContextId() ==
-            state->closest_match->GetSortingContextId() &&
-        distance_to_intersection >
-            state->closest_distance + std::numeric_limits<float>::epsilon();
+      bool in_front_of_previous_candidate =
+          state->closest_match &&
+          layer->GetSortingContextId() ==
+              state->closest_match->GetSortingContextId() &&
+          distance_to_intersection >
+              state->closest_distance + std::numeric_limits<float>::epsilon();
 
-    if (!state->closest_match || in_front_of_previous_candidate) {
-      state->closest_distance = distance_to_intersection;
-      state->closest_match = layer;
+      if (!state->closest_match || in_front_of_previous_candidate) {
+        state->closest_distance = distance_to_intersection;
+        state->closest_match = layer;
+      }
     }
+    UMA_HISTOGRAM_COUNTS("Compositing.LayerTreeImpl.FindClosestMatchingLayerUs",
+                         timer.Elapsed().InMicroseconds());
   }
 }