Support saving browser metrics to disk and reading them during next run.

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 24c104cee4f204ee8dd93f2b40a16306f7d53844..d7cf9fb8096eae0e62e218c4a210f4a90461e8e1 100644
--- a/base/metrics/histogram_base.h
+++ b/base/metrics/histogram_base.h
@@ -202,6 +202,10 @@ class BASE_EXPORT HistogramBase {
   // changed since the last call.
   virtual std::unique_ptr<HistogramSamples> SnapshotDelta() = 0;
 
+  // Calculate the difference in histogram counts since the previous call to
+  // SnapshotDelta(). Calls to this are idempotent.
+  virtual std::unique_ptr<HistogramSamples> SnapshotDifference() const = 0;

[Ilya Sherman, 2016/04/19 00:57:46]

Hmm, I think the names SnapshotDelta() vs SnapshotDifference() are too similar,
and the don't really indicate which one does what.  I'm trying to think of a
better name for this method.  Maybe SnapshotDeltaIdempotently()?  It's kind of
complicated, but at least it will nudge the reader to think a little harder
about what's going on.  It's honestly kind of a weird thing to have both methods
in this API.  I need to think more about whether/why both are needed.

[bcwhite, 2016/04/19 16:33:37]

How about "UploadDelta" (for the previously existing method) to indicate that
it's specifically for uploading to UMA?

Both are needed because sometimes metrics data is read-only and sometimes it is
read/write with each successive call returning only what has changed since the
last call.  The Histograms and the SnapshotManager don't know which.

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