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 2df75a37e81ccc35c2ff1f0acb34aa7ca1ee3116..7ebb3522923ecf71e1562f1f4eacc13328e5637e 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -6643,6 +6643,15 @@ 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>
@@ -107183,6 +107192,19 @@ value.
   <affected-histogram name="NQE.TransportRTT.Percentile90"/>
 </histogram_suffixes>
 
+<histogram_suffixes name="NumLayersBucket">
+  <suffix name="0" label="Layer count bucket 0"/>
+  <suffix name="1" label="Layer count bucket 1"/>
+  <suffix name="2" label="Layer count bucket 2"/>
+  <suffix name="3" label="Layer count bucket 3"/>
+  <suffix name="4" label="Layer count bucket 4"/>
+  <suffix name="5" label="Layer count bucket 5"/>
+  <suffix name="6" label="Layer count bucket 6"/>
+  <suffix name="7" label="Layer count bucket 7"/>
+  <suffix name="8" label="Layer count bucket 8"/>

[flackr, 2016/11/16 16:56:51]

nit: I think we should list the ranges in here.

[majidvp, 2016/12/02 20:03:31]

Done.

+  <affected-histogram name="Compositing.Renderer.LayersUpdateTime"/>
+</histogram_suffixes>
+
 <histogram_suffixes name="OffDomainInclusionAbortReason" separator=".">
   <obsolete>
     Deprecated 2016-05 as the OffDomainInclusionDetector was removed.