PerformanceMonitor: Add a new UMA histograms to track average CPU utilization

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 c89d3d39e449f91bb8e32cd68c1954f223dac1b5..23b5c43e4ff2e9a46d8e9fc315cd6b32853ef7e8 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -11282,6 +11282,13 @@ other types of suffix sets.
   </summary>
 </histogram>
 
+<histogram name="PerformanceMonitor.AverageCPU.BrowserProcess">
+  <summary>
+    Average CPU utilization of the browser process, read out each two-minute

[Yoyo Zhou, 2013/10/18 21:13:48]

s/out/at/?

[oystein (OOO til 10th of July), 2013/10/21 14:38:45]

Done.

+    interval.

[Alexei Svitkine (slow), 2013/10/18 21:39:12]

What are the units? Is it percentage? (if so, add units=%" above). Also, if it
is %, please clarify what 100% means (e.g. on a dual cpu is 50% using one core
fully or will that report as 100%?)

[Yoyo Zhou, 2013/10/18 21:49:31]

I believe it's different on different OSes, so that needs to be documented here
too.

[oystein (OOO til 10th of July), 2013/10/21 14:38:45]

Done.

[oystein (OOO til 10th of July), 2013/10/21 14:38:45]

Done.

+  </summary>
+</histogram>
+
 <histogram name="PerformanceMonitor.HighCPU.BrowserProcess" enum="BooleanHit">
   <summary>
     The number of times a browser process has continuously stayed above a