Support "shared" histograms between processes.

base/metrics/histogram_snapshot_manager.h

Index: base/metrics/histogram_snapshot_manager.h
diff --git a/base/metrics/histogram_snapshot_manager.h b/base/metrics/histogram_snapshot_manager.h
index 484287c2ab9f35b27ba53475a19e2beda9a39abb..0d2f05bf6819e15f071e87d00eda93bb7c31906a 100644
--- a/base/metrics/histogram_snapshot_manager.h
+++ b/base/metrics/histogram_snapshot_manager.h
@@ -34,8 +34,17 @@ class BASE_EXPORT HistogramSnapshotManager {
   // Only histograms that have all the flags specified by the argument will be
   // chosen. If all histograms should be recorded, set it to
   // |Histogram::kNoFlags|.
-  void PrepareDeltas(HistogramBase::Flags flags_to_set,
-                     HistogramBase::Flags required_flags);
+  template <class ForwardHistogramIterator>
+  void PrepareDeltas(ForwardHistogramIterator begin,
+                     ForwardHistogramIterator end,
+                     HistogramBase::Flags flags_to_set,
+                     HistogramBase::Flags required_flags) {
+    for (ForwardHistogramIterator it = begin; it != end; ++it) {
+      (*it)->SetFlags(flags_to_set);
+      if (((*it)->flags() & required_flags) == required_flags)
+        PrepareDelta(**it);
+    }
+  }
 
  private:
   // Snapshot this histogram, and record the delta.