Enable UMA histogram logger for low memor kill events in ArcMetricsService.

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 b42f34c6a2b2dd16b1070c699d0f2899c5dab4dd..8accfe12bc68edd4087a00114f6eb9442d862d71 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -835,12 +835,33 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Arc.LowMemoryKiller.Count">

[hidehiko, 2016/03/24 08:11:21]

It's better to have a description in your commit message.

[cylee1, 2016/03/24 14:36:07]

Done.

[hidehiko, 2016/03/24 16:52:25]

Oops, sorry for my poor explanation. Would you mind also to mention about the
renaming from ArcRuntime to Arc?

+  <owner>elijahtaylor@google.com</owner>
+  <summary>Cumulative count of low memory kills in one user session.</summary>
+</histogram>
+
+<histogram name="Arc.LowMemoryKiller.FreedSize" units="KB">
+  <owner>elijahtaylor@google.com</owner>
+  <summary>The memory size freed by each low memory kill event.</summary>
+</histogram>
+
+<histogram name="Arc.LowMemoryKiller.TimeDelta" units="ms">
+  <owner>elijahtaylor@google.com</owner>
+  <summary>The elapsed time to last low memory kill event.</summary>
+</histogram>
+
 <histogram name="ArcRuntime.LowMemoryKiller.FreedSize" units="KB">
+  <obsolete>
+    Renamed to Arc.LowMemoryKiller.FreedSize on 2016/03/24.
+  </obsolete>
   <owner>elijahtaylor@google.com</owner>
   <summary>The memory size freed by each low memory kill event.</summary>
 </histogram>
 
 <histogram name="ArcRuntime.LowMemoryKiller.TimeDelta" units="ms">
+  <obsolete>
+    Renamed to Arc.LowMemoryKiller.TimeDelta on 2016/03/24.
+  </obsolete>
   <owner>elijahtaylor@google.com</owner>
   <summary>The elapsed time to last low memory kill event.</summary>
 </histogram>