Merge multiple histogram snapshots into single one for reporting.

components/metrics/metrics_service.h

Index: components/metrics/metrics_service.h
diff --git a/components/metrics/metrics_service.h b/components/metrics/metrics_service.h
index ce4102b8dfa36408fe037bd6772184d759a48a4e..e5d79bcf3e276b5735dfe511c6f8ad7085108978 100644
--- a/components/metrics/metrics_service.h
+++ b/components/metrics/metrics_service.h
@@ -11,6 +11,7 @@
 #include <stdint.h>
 
 #include <map>
+#include <set>
 #include <string>
 #include <vector>
 
@@ -73,6 +74,8 @@ class MetricsService : public base::HistogramFlattener {
     SHUTDOWN_COMPLETE = 700,
   };
 
+  typedef std::set<base::HistogramBase*> HistogramSet;
+
   // Creates the MetricsService with the given |state_manager|, |client|, and
   // |local_state|.  Does not take ownership of the paramaters; instead stores
   // a weak pointer to each. Caller should ensure that the parameters are valid