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 0a3983f8eabb8fedb0a2a1be666ae323f294fa89..2ddbc8643820ce462efc29aca4a59dc27c5d95ae 100644
--- a/base/metrics/histogram_delta_serialization.h
+++ b/base/metrics/histogram_delta_serialization.h
@@ -29,8 +29,11 @@ class BASE_EXPORT HistogramDeltaSerialization : public HistogramFlattener {
   // Computes deltas in histogram bucket counts relative to the previous call to
   // this method. Stores the deltas in serialized form into |serialized_deltas|.
   // If |serialized_deltas| is NULL, no data is serialized, though the next call

[Alexei Svitkine (slow), 2016/01/18 19:25:35]

Nit: "is NULL" -> "is null" per new conventions since you're updating the
comment.

[bcwhite, 2016/01/22 15:24:53]

Done.

-  // will compute the deltas relative to this one.
-  void PrepareAndSerializeDeltas(std::vector<std::string>* serialized_deltas);
+  // will compute the deltas relative to this one. Setting |include_persistent|
+  // will include histograms held in persistent memory (and thus may be reported
+  // elsewhere); otherwise only histograms local to this process are serialized.
+  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|.