Add TaskScheduler.NumTasksBeforeDetach.[worker pool name] 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 c38d839599ed3e0c43a57e9ee4192fcb9c4dedd7..3425e4910ed435e9b1444c1f310de13bda17090d 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -64325,6 +64325,17 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="TaskScheduler.NumTasksBeforeDetach" 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 before it detached. Recorded
+    when the SchedulerWorker is woken up after detaching. A low value indicates
+    that SchedulerWorkers are detached too often.

[robliao, 2016/10/13 18:04:04]

Nit: Another conclusion could be the rate of work is low. Let's remove the last
sentence.

[fdoray, 2016/10/13 20:38:53]

Done.

+  </summary>
+</histogram>
+
 <histogram name="TaskScheduler.NumTasksBetweenWaits" units="tasks">
   <owner>fdoray@chromium.org</owner>
   <owner>gab@chromium.org</owner>
@@ -110082,6 +110093,7 @@ 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.NumTasksBeforeDetach"/>
   <affected-histogram name="TaskScheduler.NumTasksBetweenWaits"/>
   <affected-histogram name="TaskScheduler.TaskLatency"/>
 </histogram_suffixes>