cc: Record UMA for swap to swap ack latency.

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 6e5930812d6d766fd4736618d7e70af3f9050d35..7ba5ac97064c28fe8ab80a3fa2f984cd05ea6bee 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -40718,6 +40718,11 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Scheduling.SwapToAckLatency" units="microseconds">
+  <owner>brianderson@chromium.org</owner>
+  <summary>How long it takes the swap ack to return after a swap.</summary>
+</histogram>
+
 <histogram name="Sdch3.AdvertisedWithSecureScheme" enum="BooleanHttps">
   <obsolete>
     Experiment complete, histogram gathering code removed.
@@ -76716,6 +76721,7 @@ To add a new entry, add it with any value and run test to compute valid value.
   <affected-histogram name="Scheduling.CommitToReadyToActivateDuration"/>
   <affected-histogram name="Scheduling.DrawDuration"/>
   <affected-histogram name="Scheduling.PrepareTilesDuration"/>
+  <affected-histogram name="Scheduling.SwapToAckLatency"/>
 </histogram_suffixes>
 
 <histogram_suffixes name="CompositorTimingHistoryProcess2" separator="."