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

base/process/process_metrics.h

Index: base/process/process_metrics.h
diff --git a/base/process/process_metrics.h b/base/process/process_metrics.h
index f6b225fa94fab2603fdcc51ab425e9dadcc90a4e..326338781f99b3d001fcc92521df9e9c8b8fc5f0 100644
--- a/base/process/process_metrics.h
+++ b/base/process/process_metrics.h
@@ -168,6 +168,10 @@ class BASE_EXPORT ProcessMetrics {
   // method returns 50.
   double GetCPUUsage();
 
+  // Same as GetCPUUsage(), but will return consistent returns on all platforms
+  // including Windows.

[Alexei Svitkine (slow), 2013/10/24 14:52:00]

Mention what the consistent values actually are.

[oystein (OOO til 10th of July), 2013/10/24 15:01:06]

Actually meant to write "values" rather than "returns" (fixed now), but
otherwise do you mean basically repeating the comment for GetCPUUsage() above?

[Alexei Svitkine (slow), 2013/10/24 15:13:33]

Not necessarily repeating, you can refer to it - just make it clear that it
makes Windows consistent with other platforms and not vice versa, but being
super explicit is even better (e.g. specify that 1 core maxed is 100% and 2
cores maxed is 200%, etc.)

Also, now that I look at the comment above, it says "Returns the CPU usage in
percent since the last time this method was called." Probably now it should say
"since the last GetCPUUsage() or GetPlatformIndepententCPUUsage() call".

[oystein (OOO til 10th of July), 2013/10/24 15:25:51]

Good point; done.

+  double GetPlatformIndependentCPUUsage();
+
   // Retrieves accounting information for all I/O operations performed by the
   // process.
   // If IO information is retrieved successfully, the function returns true