Add UMA metric Browser.Responsiveness.JankyIntervalsPerThirtySeconds

All the logic was hooked up for calculation of the metric but the UMA metric
itself was missing.

Bug: 859155
Change-Id: Iab4e3c726e2627d1f1325ee30bebd1ee9eae6f76
Reviewed-on: https://chromium-review.googlesource.com/1179985Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Timothy Dresser <tdresser@chromium.org>
Commit-Queue: Erik Chen <erikchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584337}

content/browser/scheduler/responsiveness/calculator.cc

@@ -7,6 +7,7 @@
 #include <algorithm>
 #include <set>
 
+#include "base/metrics/histogram_macros.h"
 #include "content/public/browser/browser_thread.h"
 
 namespace content {
@@ -86,7 +87,8 @@ void Calculator::TaskOrEventFinishedOnIOThread(base::TimeTicks schedule_time,
 }
 
 void Calculator::EmitResponsiveness(int janky_slices) {
-  // TODO(erikchen): Emit an UMA metric. https://crbug.com/859155.
+  UMA_HISTOGRAM_COUNTS_1000(
+      "Browser.Responsiveness.JankyIntervalsPerThirtySeconds", janky_slices);
 }
 
 base::TimeTicks Calculator::GetLastCalculationTime() {

tools/metrics/histograms/histograms.xml

@@ -10427,6 +10427,19 @@ uploading your change for review.
   <summary>Maximal amount of memory allocated by decoder.</summary>
 </histogram>
 
+<histogram name="Browser.Responsiveness.JankyIntervalsPerThirtySeconds"
+    units="janks" expires_after="M75">
+  <owner>erikchen@chromium.org</owner>
+  <owner>tdresser@chromium.org</owner>
+  <summary>
+    This metric is emitted every 30 seconds [when there is user activity]. Each
+    30 second duration is divided into 100ms intervals. This metric counts the
+    number of these intervals that were &quot;janky&quot;. See
+    https://docs.google.com/document/d/1vDSGFvJblh7yJ3U3RVB_7qZLubyfTbQdQjuN1GoUNkc/edit
+    for more details.
+  </summary>
+</histogram>
+
 <histogram name="Browser.Tabs.SelectionToVisibilityRequestTime"
     units="microseconds">
   <owner>sadrul@chromium.org</owner>