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

chrome/browser/performance_monitor/metric.h

Index: chrome/browser/performance_monitor/metric.h
diff --git a/chrome/browser/performance_monitor/metric.h b/chrome/browser/performance_monitor/metric.h
index 76e8ded8d814ad2b38fdf90daa6780a84e35745a..79b9ed7c75e644180a774374408c6e875e86fc8a 100644
--- a/chrome/browser/performance_monitor/metric.h
+++ b/chrome/browser/performance_monitor/metric.h
@@ -32,10 +32,25 @@ METRIC_NUMBER_OF_METRICS
 struct Metric {
  public:
   Metric();
-  Metric(const base::Time& metric_time, const double metric_value);
-  Metric(const std::string& metric_time, const std::string& metric_value);
+  Metric(MetricType metric_type,
+         const base::Time& metric_time,
+         const double metric_value);
+  Metric(MetricType metric_type,
+         const std::string& metric_time,
+         const std::string& metric_value);
   ~Metric();
 
+  // Check the value in the metric to make sure that it is reasonable. Since
+  // some metric-gathering methods will fail and return incorrect values, we
+  // need to try to weed these out as best we can.
+  bool IsValid() const;
+
+  // This converts the double stored in value to a string format. This will
+  // not perform any checking on the validity of the metric, and only makes
+  // sense if the metric IsValid().
+  std::string ValueAsString() const;
+
+  MetricType type;
   base::Time time;
   double value;
 };