Merge multiple histogram snapshots into single one for reporting.

base/metrics/histogram_base.h

Index: base/metrics/histogram_base.h
diff --git a/base/metrics/histogram_base.h b/base/metrics/histogram_base.h
index 40c2caedc313a815c6400b9893feb093c1a6d676..539bd5f885e7968469bc7ee493cec79e966423e7 100644
--- a/base/metrics/histogram_base.h
+++ b/base/metrics/histogram_base.h
@@ -160,6 +160,9 @@ class BASE_EXPORT HistogramBase {
   // Override with atomic/locked snapshot if needed.
   virtual scoped_ptr<HistogramSamples> SnapshotSamples() const = 0;
 
+  // Calculate delta from the previous call to this method.

[Alexei Svitkine (slow), 2016/02/09 19:46:51]

I don't think the comment is sufficiently clear. Please expand to make it clear
what it's doing.

"delta from previous call to this method" doesn't tell me much - i.e. delta of
what?

[bcwhite, 2016/02/11 16:42:38]

Done.

+  virtual scoped_ptr<HistogramSamples> SnapshotDelta() = 0;
+
   // The following methods provide graphical histogram displays.
   virtual void WriteHTMLGraph(std::string* output) const = 0;
   virtual void WriteAscii(std::string* output) const = 0;