Support "shared" histograms between processes.

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 3396829b43de14389839bb6185090656209f5137..bb9949df25b8259c2d1078a3279879bbe2a6cb46 100644
--- a/base/metrics/statistics_recorder.h
+++ b/base/metrics/statistics_recorder.h
@@ -34,6 +34,33 @@ class BASE_EXPORT StatisticsRecorder {
  public:
   typedef std::vector<HistogramBase*> Histograms;
 
+  // A class for iterating over the histograms held within this global resource.
+  class BASE_EXPORT HistogramIterator {
+   public:
+    HistogramIterator(HistogramMap::iterator iter, bool include_persistent)
+        : iter_(iter),
+          include_persistent_(include_persistent) {}
+
+    HistogramIterator& operator++();
+    HistogramIterator operator++(int) {
+      HistogramIterator tmp(*this);
+      operator++();
+      return tmp;
+    }
+
+    bool operator==(const HistogramIterator& rhs) const {
+      return iter_ == rhs.iter_;
+    }
+    bool operator!=(const HistogramIterator& rhs) const {
+      return iter_ != rhs.iter_;
+    }
+    HistogramBase* operator*() { return iter_->second; }
+
+   private:
+    HistogramMap::iterator iter_;
+    const bool include_persistent_;
+  };
+
   // Initializes the StatisticsRecorder system. Safe to call multiple times.
   static void Initialize();
 
@@ -73,6 +100,10 @@ class BASE_EXPORT StatisticsRecorder {
   // safe.  It returns NULL if a matching histogram is not found.
   static HistogramBase* FindHistogram(const std::string& name);
 
+  // Support for iterating over known histograms.
+  static HistogramIterator begin(bool include_persistent);
+  static HistogramIterator end();
+
   // GetSnapshot copies some of the pointers to registered histograms into the
   // caller supplied vector (Histograms). Only histograms which have |query| as
   // a substring are copied (an empty string will process all registered
@@ -112,6 +143,7 @@ class BASE_EXPORT StatisticsRecorder {
   friend class HistogramSnapshotManagerTest;
   friend class HistogramTest;
   friend class JsonPrefStoreTest;
+  friend class SharedHistogramTest;
   friend class SparseHistogramTest;
   friend class StatisticsRecorderTest;
   FRIEND_TEST_ALL_PREFIXES(HistogramDeltaSerializationTest,