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

base/process/process_metrics.cc

Index: base/process/process_metrics.cc
diff --git a/base/process/process_metrics.cc b/base/process/process_metrics.cc
index 127fb4649f70b04f054db74b9f97b4132173d1fa..83289b8c78bf06c2ecc5f0a1f9bc479a5629f663 100644
--- a/base/process/process_metrics.cc
+++ b/base/process/process_metrics.cc
@@ -42,4 +42,12 @@ scoped_ptr<Value> SystemMetrics::ToValue() const {
   return res.PassAs<Value>();
 }
 
+double ProcessMetrics::GetPlatformIndependentCPUUsage() {
+#if defined(OS_WIN)
+  return GetCPUUsage() * processor_count_;
+#else
+  return GetCPUUsage();
+#endif
+}
+
 }  // namespace base