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..58a9ab2edc093393fdd7177c3f99f47b543b41e4 100644
--- a/base/metrics/histogram_base.h
+++ b/base/metrics/histogram_base.h
@@ -202,6 +202,15 @@ class BASE_EXPORT HistogramBase {
   // changed since the last call.
   virtual std::unique_ptr<HistogramSamples> SnapshotDelta() = 0;
 
+  // Calculate the change (delta) in histogram counts since the previous call
+  // to SnapshotDelta() but do so without modifying any internal data as to
+  // what was previous logged. After such a call, no further calls to this
+  // method or to SnapshotDelta() should be done as the result would include
+  // data previously returned. Because no internal data is changed, this call
+  // can be made on "const" histograms such as those with data held in
+  // read-only memory.
+  virtual std::unique_ptr<HistogramSamples> SnapshotFinalDelta() const = 0;
+
   // The following methods provide graphical histogram displays.
   virtual void WriteHTMLGraph(std::string* output) const = 0;
   virtual void WriteAscii(std::string* output) const = 0;