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 b8b453b8f84f0f5fed652c8bcd00ba12e04a1072..a3328cb13ff2b5f25c39562f9a5cd3e8cc76a5bf 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -58064,6 +58064,26 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <summary>Whether a scroll is executed on main thread.</summary>
 </histogram>
 
+<histogram name="UMA.ActivityTracker.ThreadTrackers.Count">
+  <owner>bcwhite@chromium.org</owner>
+  <summary>
+    The number of threads being tracked for activities. 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="UMA.ActivityTracker.ThreadTrackers.MemLimit">
+  <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="UMA.ActualLogUploadInterval" units="minutes">
   <owner>asvitkine@chromium.org</owner>
   <summary>