Add TaskScheduler.NumTasksBetweenWaits.[pool] histogram.

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 765057bf1a826efaf038ddec1f4349aec05f0568..ad92eb150160d0e4e3e1ac611a70641925f8f403 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -62584,6 +62584,16 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="TaskScheduler.NumTasksBetweenWaits" units="tasks">
+  <owner>fdoray@chromium.org</owner>
+  <owner>gab@chromium.org</owner>
+  <owner>robliao@chromium.org</owner>
+  <summary>
+    Number of tasks executed by a SchedulerWorker between two waits on its
+    WaitableEvent.

[gab, 2016/09/22 18:43:45]

Add to the description that the goal is to maximize this without affecting
TaskScheduler.TaskLatency.*

[fdoray, 2016/09/22 19:24:46]

Done.

+  </summary>
+</histogram>
+
 <histogram name="TaskScheduler.TaskLatency" units="ms">
   <owner>fdoray@chromium.org</owner>
   <owner>gab@chromium.org</owner>
@@ -106841,6 +106851,7 @@ To add a new entry, add it with any value and run test to compute valid value.
       label="Applies to the ForegroundFileIO worker pool."/>
   <suffix name="ForegroundPool" label="Applies to the Foreground worker pool."/>
   <affected-histogram name="TaskScheduler.DetachDuration"/>
+  <affected-histogram name="TaskScheduler.NumTasksBetweenWaits"/>
   <affected-histogram name="TaskScheduler.TaskLatency"/>
 </histogram_suffixes>