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 705cdbcf1022b5c30e14a6f7d42eae37a5bc92f0..ef8dd07ad68bcd6cb5fc4adfeea52e02e82cf5dd 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -324,6 +324,29 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="ActivityTracker.ThreadTrackers.Count" units="count">
+  <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="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="Android.Activity.ChromeTabbedActivity.StopReason"
     enum="AndroidActivityStopReason">
   <owner>dfalcantara@chromium.org</owner>