Track thread activities in order to diagnose hangs.

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 06532ba140819f137bad3e921a9721d53e544c64..c12112b358318db22b97cff6336b665011b86ede 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -8259,6 +8259,29 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Debug.ActivityTracker.ThreadTrackers.Count" units="count">

[Alexei Svitkine (slow), 2016/08/04 18:16:45]

Okay, in this case remove the Debug. prefix.

[bcwhite, 2016/08/04 19:15:07]

Done.

+  <owner>bcwhite@chromium.org</owner>
+  <summary>
+    The number of threads being tracked for activities such as waiting for a
+    lock/event, a thread to join, or a task to run. Analysis of this data can
+    reveal why a thread will not exit. This value is updated every time a new
+    thread gets an activity tracked for the first time so will show one count
+    for every permanent thread but multiple counts for a thread than exits and
+    is replaced.
+  </summary>
+</histogram>
+
+<histogram name="Debug.ActivityTracker.ThreadTrackers.MemLimitTrackerCount"
+    units="count">
+  <owner>bcwhite@chromium.org</owner>
+  <summary>
+    The limit on the number of thread trackers that could be allocated from the
+    persistent memory segment. Trackers beyond this number were allocated from
+    the heap and thus were not available for analysis. This value is updated on
+    every &quot;memory full&quot; failure.
+  </summary>
+</histogram>
+
 <histogram name="DefaultBrowser.ActivateSettings.ErrorHresult" enum="Hresult">
   <owner>pmonette@chromium.org</owner>
   <owner>grt@chromium.org</owner>