Support "shared" histograms between processes.

base/metrics/histogram_delta_serialization.h

Index: base/metrics/histogram_delta_serialization.h
diff --git a/base/metrics/histogram_delta_serialization.h b/base/metrics/histogram_delta_serialization.h
index 0bc4a5905fb8470cbd1c9884da006b6cdf57209e..ae8aa9095e394a6ef326bfcd647b3fa92ed8929b 100644
--- a/base/metrics/histogram_delta_serialization.h
+++ b/base/metrics/histogram_delta_serialization.h
@@ -30,7 +30,8 @@ class BASE_EXPORT HistogramDeltaSerialization : public HistogramFlattener {
   // this method. Stores the deltas in serialized form into |serialized_deltas|.
   // If |serialized_deltas| is NULL, no data is serialized, though the next call
   // will compute the deltas relative to this one.
-  void PrepareAndSerializeDeltas(std::vector<std::string>* serialized_deltas);
+  void PrepareAndSerializeDeltas(std::vector<std::string>* serialized_deltas,
+                                 bool include_persistent);
 
   // Deserialize deltas and add samples to corresponding histograms, creating
   // them if necessary. Silently ignores errors in |serialized_deltas|.