Support "shared" histograms between processes.

content/child/child_histogram_message_filter.cc

Index: content/child/child_histogram_message_filter.cc
diff --git a/content/child/child_histogram_message_filter.cc b/content/child/child_histogram_message_filter.cc
index dbf2dd22be18a5f9fda7a7425d64d40214380f55..3190a32e569b16808316855442c3b4f011845638 100644
--- a/content/child/child_histogram_message_filter.cc
+++ b/content/child/child_histogram_message_filter.cc
@@ -59,7 +59,10 @@ void ChildHistogramMessageFilter::UploadAllHistograms(int sequence_number) {
   }
 
   std::vector<std::string> deltas;
-  histogram_delta_serialization_->PrepareAndSerializeDeltas(&deltas);
+  // "false" to SPerpareAndSerializeDeltas() indicates to *not* include
+  // histograms held in persistent storage on the assumption that they will be
+  // visible to the recipient through other means.
+  histogram_delta_serialization_->PrepareAndSerializeDeltas(&deltas, false);
   sender_->Send(
       new ChildProcessHostMsg_ChildHistogramData(sequence_number, deltas));