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