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 4b9a8a12064234213ce7541646524a448753c10d..e4873591a783ae9f276f0dca67d6a1a458560ae5 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -51659,6 +51659,15 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Scheduling.SwapAckWasFast" enum="BooleanWasFast">
+  <owner>brianderson@chromium.org</owner>
+  <summary>
+    True if the swap ack was received within approximately 8 seconds of the
+    swap. Although 8 seconds isn't exactly fast, it is a threshold that
+    represents a hang or the appearance of a hang.
+  </summary>
+</histogram>
+
 <histogram name="Scheduling.SwapToAckLatency" units="microseconds">
   <obsolete>
     Replaced by SwapToAckLatency2, due to inefficient bucketing scheme.
@@ -70355,6 +70364,11 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <int value="1" label="Virtual context encountered"/>
 </enum>
 
+<enum name="BooleanWasFast" type="int">
+  <int value="0" label="Slow"/>
+  <int value="1" label="Fast"/>
+</enum>
+
 <enum name="BooleanWasted" type="int">
   <int value="0" label="Not wasted"/>
   <int value="1" label="Wasted"/>
@@ -98227,6 +98241,7 @@ 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>