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 752b6131947870e3170b0851340c0b75c0858cfb..bea2bf9c9df914e9ad119843d49f686b055ec2b9 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -62919,6 +62919,17 @@ 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. This should be maximized without affecting perceived browser
+    performance.
+  </summary>
+</histogram>
+
 <histogram name="TaskScheduler.TaskLatency" units="ms">
   <owner>fdoray@chromium.org</owner>
   <owner>gab@chromium.org</owner>
@@ -107947,6 +107958,7 @@ 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>