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 bad4668067b04f3fb3d17474e52b35bf3967aa90..d1ba2a5002a88273c7e88b449f0f7f4a1be07cd3 100644
--- a/base/metrics/histogram_snapshot_manager.h
+++ b/base/metrics/histogram_snapshot_manager.h
@@ -36,8 +36,17 @@ 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.