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..81fd44517a6665406369871910be75eeff2236cd 100644
--- a/base/process/process_metrics.h
+++ b/base/process/process_metrics.h
@@ -160,14 +160,19 @@ class BASE_EXPORT ProcessMetrics {
   // load and fragmentation.
   bool CalculateFreeMemory(FreeMBytes* free) const;
 
-  // Returns the CPU usage in percent since the last time this method was
-  // called. The first time this method is called it returns 0 and will return
-  // the actual CPU info on subsequent calls.
-  // On Windows, the CPU usage value is for all CPUs. So if you have 2 CPUs and
-  // your process is using all the cycles of 1 CPU and not the other CPU, this
-  // method returns 50.
+  // Returns the CPU usage in percent since the last time this method or
+  // GetPlatformIndependentCPUUsage() was called. The first time this method
+  // is called it returns 0 and will return the actual CPU info on subsequent
+  // calls. On Windows, the CPU usage value is for all CPUs. So if you have
+  // 2 CPUs and your process is using all the cycles of 1 CPU and not the other
+  // CPU, this method returns 50.
   double GetCPUUsage();
 
+  // Same as GetCPUUsage(), but will return consistent values on all platforms
+  // (cancelling the Windows exception mentioned above) by returning a value in
+  // the range of 0 to (100 * numCPUCores) everywhere.
+  double GetPlatformIndependentCPUUsage();
+
   // Retrieves accounting information for all I/O operations performed by the
   // process.
   // If IO information is retrieved successfully, the function returns true