Record the fraction of the time without user input - Aura only

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 b764a3e8a9478a894526d07f083132ef49564911..c107145e7061623ae535213eb5c6213ac58da2eb 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -15307,6 +15307,16 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Event.FractionOfTimeWithoutUserInput">

[Ilya Sherman, 2017/03/27 19:07:57]

nit: Please add a units attribute.

[tdresser, 2017/03/27 19:34:27]

Done.

+  <owner>tdresser@chromium.org</owner>
+  <owner>progressive-web-metrics@chromium.org</owner>
+  <summary>
+    For each 10 second window, reports the fraction of the time there was no
+    user input. We consider there to be no user input if we haven't received any
+    events for &gt; 50ms.
+  </summary>
+</histogram>
+
 <histogram name="Event.FrameEventRouting.NoGestureTarget">
   <owner>wjmaclean@chromium.org</owner>
   <summary>