UMA metric for LayerUpdateTimes

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 41da607a2a839b3ac49e765536be5bbe99f622d7..a48ac821a5659d519a3e046515e34576b9376204 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -6846,6 +6846,15 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Compositing.Renderer.LayersUpdateTime" units="microseconds">
+  <owner>threaded-rendering@chromium.org</owner>
+  <summary>
+    Time spent updating layers, in microseconds. Recorded when layers are
+    updated (in a renderer process). Suffixed with a number that denotes number
+    of layers / 50.
+  </summary>
+</histogram>
+
 <histogram name="Compositing.Renderer.LayerTreeImpl.CalculateDrawPropertiesUs"
     units="microseconds">
   <owner>paint-dev@chromium.org</owner>
@@ -110953,6 +110962,15 @@ value.
   <affected-histogram name="NQE.TransportRTT.Percentile90"/>
 </histogram_suffixes>
 
+<histogram_suffixes name="NumLayersBucket">
+  <suffix name="0" label="Layer count bucket [0, 10)"/>
+  <suffix name="1" label="Layer count bucket [10, 30)"/>
+  <suffix name="2" label="Layer count bucket [30, 70)"/>
+  <suffix name="3" label="Layer count bucket [70, 150)"/>
+  <suffix name="4" label="Layer count bucket [150, infinity)"/>
+  <affected-histogram name="Compositing.Renderer.LayersUpdateTime"/>
+</histogram_suffixes>
+
 <histogram_suffixes name="OffDomainInclusionAbortReason" separator=".">
   <obsolete>
     Deprecated 2016-05 as the OffDomainInclusionDetector was removed.