cc: Add MainAndImplFrameTimeDelta UMA.

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 1166704b12137b3978febe125bff5415b38fef1d..c04f242b1716f9562a49801d5f407f5e4a418ebb 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -43012,6 +43012,18 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Scheduling.MainAndImplFrameTimeDelta" units="microseconds">
+  <owner>brianderson@chromium.org</owner>
+  <summary>
+    Recorded every time the compositor draws with a new active tree. A value of
+    0 indicates the main-side started and finished within the same frame
+    interval as the impl-side. Positive values correspond to how old any
+    main-side updates are compared to the impl-side updates. If there are no
+    mid-frame updates, this metric is a good proxy for how well the main and
+    impl threads are synchronized.
+  </summary>
+</histogram>
+
 <histogram name="Scheduling.PrepareTilesDuration" units="microseconds">
   <owner>brianderson@chromium.org</owner>
   <summary>
@@ -82243,6 +82255,7 @@ To add a new entry, add it with any value and run test to compute valid value.
   <affected-histogram name="Scheduling.BeginMainFrameToCommitDuration"/>
   <affected-histogram name="Scheduling.CommitToReadyToActivateDuration"/>
   <affected-histogram name="Scheduling.DrawDuration"/>
+  <affected-histogram name="Scheduling.MainAndImplFrameTimeDelta"/>
   <affected-histogram name="Scheduling.PrepareTilesDuration"/>
   <affected-histogram name="Scheduling.SwapToAckLatency"/>
 </histogram_suffixes>