Optimize memory use of registered histograms.

base/metrics/statistics_recorder.h

Index: base/metrics/statistics_recorder.h
diff --git a/base/metrics/statistics_recorder.h b/base/metrics/statistics_recorder.h
index b5230575963e079789fac0f36c22fd2be5ba5ae5..545c4663c529aa3567b0b402813f9f62b3d854d4 100644
--- a/base/metrics/statistics_recorder.h
+++ b/base/metrics/statistics_recorder.h
@@ -76,8 +76,24 @@ class BASE_EXPORT StatisticsRecorder {
   static void GetSnapshot(const std::string& query, Histograms* snapshot);
 
  private:
+  // HistogramNameRef holds a weak const ref to the name field of the associated
+  // Histogram object, allowing re-use of the underlying string storage for the
+  // map keys. The wrapper is required as using "const std::string&" as the key
+  // results in compile errors.
+  struct HistogramNameRef {
+    explicit HistogramNameRef(const std::string& name) : name_(name) {};
+
+    // Operator < is necessary to use this type as a std::map key.
+    bool operator<(const HistogramNameRef& other) const {
+      return name_ < other.name_;
+    }
+
+    // Weak, owned by the associated Histogram object.
+    const std::string& name_;
+  };
+
   // We keep all registered histograms in a map, from name to histogram.
-  typedef std::map<std::string, HistogramBase*> HistogramMap;
+  typedef std::map<HistogramNameRef, HistogramBase*> HistogramMap;
 
   // We keep all |bucket_ranges_| in a map, from checksum to a list of
   // |bucket_ranges_|.  Checksum is calculated from the |ranges_| in