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 f335cdf74bb2f4146a848774cf2056530c3ad0fd..7a2e63df2e1a67d1fd985a2fbd239008e44f4bcd 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -11336,6 +11336,17 @@ other types of suffix sets.
   </summary>
 </histogram>
 
+<histogram name="PerformanceMonitor.AverageCPU.BrowserProcess">

[jar (doing other things), 2013/10/22 22:30:03]

nit; add a units tag, which is probably "PercentCpuUsage"

[oystein (OOO til 10th of July), 2013/10/24 14:47:56]

Done.

+  <summary>
+    Average CPU utilization of the browser process, read out at each two-minute
+    interval. On Windows this is a range from 0-100% of the total CPU capacity,
+    i.e. a quadcore system with one core running fully loaded and the other
+    three idle would read as 25%. On non-Windows systems there's a 0-100% range
+    per CPU, which is then summed up. I.e. a quadcore system fully loaded would
+    read as 400%.
+  </summary>
+</histogram>
+
 <histogram name="PerformanceMonitor.HighCPU.BrowserProcess" enum="BooleanHit">
   <summary>
     The number of times a browser process has continuously stayed above a