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

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index 01cd2358bd35ba5ad2a6e38a21d3cb161b54b930..ce0c33aec63b7c8cc26a7b76a8172c9df2efed7c 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -6963,6 +6963,16 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Compositing.Browser.LayerTreeImpl.FindClosestMatchingLayerUs"
+    units="microseconds">
+  <owner>paint-dev@chromium.org</owner>
+  <summary>
+    Time spent finding the closest matching layer to a given point, in
+    microseconds. Recorded each time we do hit testing on LayerTreeImpl (in a
+    browser process).
+  </summary>
+</histogram>
+
 <histogram name="Compositing.Browser.NumActiveLayers" units="layers">
   <owner>paint-dev@chromium.org</owner>
   <summary>
@@ -7236,6 +7246,16 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Compositing.Renderer.LayerTreeImpl.FindClosestMatchingLayerUs"
+    units="microseconds">
+  <owner>paint-dev@chromium.org</owner>
+  <summary>
+    Time spent finding the closest matching layer to a given point, in
+    microseconds. Recorded each time we do hit testing on LayerTreeImpl (in a
+    renderer process).
+  </summary>
+</histogram>
+
 <histogram name="Compositing.Renderer.NumActiveLayers" units="layers">
   <owner>paint-dev@chromium.org</owner>
   <summary>