Telemetry: Don't silently compare histograms from different processes.

base/metrics/histogram_base.cc

Index: base/metrics/histogram_base.cc
diff --git a/base/metrics/histogram_base.cc b/base/metrics/histogram_base.cc
index 835a05b56efd4ad1087e1363cd4757c1f44f7899..5272d0390af02f7e9ad18b314358080ff3420b2d 100644
--- a/base/metrics/histogram_base.cc
+++ b/base/metrics/histogram_base.cc
@@ -13,6 +13,7 @@
 #include "base/metrics/histogram_samples.h"
 #include "base/metrics/sparse_histogram.h"
 #include "base/pickle.h"
+#include "base/process_util.h"
 #include "base/values.h"
 
 namespace base {
@@ -119,6 +120,7 @@ void HistogramBase::WriteJSON(std::string* output) const {
   root.SetInteger("flags", flags());
   root.Set("params", parameters.release());
   root.Set("buckets", buckets.release());
+  root.SetInteger("pid", GetCurrentProcId());
   serializer.Serialize(root);
 }