cc: Add a Scheduling.SwapAckWasFast 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 705cdbcf1022b5c30e14a6f7d42eae37a5bc92f0..cd11fb6c4dd50d5c4443db34c37ff8689571601e 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -50670,6 +50670,14 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Scheduling.SwapAckWasFast" units="boolean">

[Ilya Sherman, 2016/08/19 23:37:56]

Please provide a custom BooleanWasFast enum, rather than specifying a units
attribute.  (Only need to make changes in histograms.xml -- no C++ changes
needed.)

+  <owner>brianderson@chromium.org</owner>
+  <summary>
+    True if the swap ack was received within approximately 8 seconds of the
+    swap.

[Ilya Sherman, 2016/08/19 23:37:56]

It would be great to mention within this description why roughly 8 seconds is
"fast".

+  </summary>
+</histogram>
+
 <histogram name="Scheduling.SwapToAckLatency" units="microseconds">
   <obsolete>
     Replaced by SwapToAckLatency2, due to inefficient bucketing scheme.
@@ -96780,6 +96788,7 @@ To add a new entry, add it with any value and run test to compute valid value.
   <affected-histogram name="Scheduling.DrawInterval2"/>
   <affected-histogram name="Scheduling.MainAndImplFrameTimeDelta2"/>
   <affected-histogram name="Scheduling.PrepareTilesDuration2"/>
+  <affected-histogram name="Scheduling.SwapAckWasFast"/>
   <affected-histogram name="Scheduling.SwapToAckLatency2"/>
 </histogram_suffixes>