Add guards to metric values; erase bad events/metrics from db

chrome/browser/performance_monitor/performance_monitor.h

Index: chrome/browser/performance_monitor/performance_monitor.h
diff --git a/chrome/browser/performance_monitor/performance_monitor.h b/chrome/browser/performance_monitor/performance_monitor.h
index a51253c11829b26a101b3007996e2157b82bd9ce..0dd1ddab32d845f7c02079bd6f427c52e49a7d1d 100644
--- a/chrome/browser/performance_monitor/performance_monitor.h
+++ b/chrome/browser/performance_monitor/performance_monitor.h
@@ -131,8 +131,9 @@ class PerformanceMonitor : public content::NotificationObserver {
   void AddEventOnBackgroundThread(scoped_ptr<Event> event);
 
   // Since Database::AddMetric() is overloaded, base::Bind() does not work and
-  // we need a helper function.
-  void AddMetricOnBackgroundThread(MetricType type, const std::string& value);
+  // we need a helper function. Deliberately not const & so that we will
+  // construct a new metric on the background thread.
+  void AddMetricOnBackgroundThread(Metric metric);
 
   // Notify any listeners that PerformanceMonitor has finished the initializing.
   void NotifyInitialized();