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

components/metrics/metrics_provider.h

Index: components/metrics/metrics_provider.h
diff --git a/components/metrics/metrics_provider.h b/components/metrics/metrics_provider.h
index 664e5dbc539299c9e9682378901e558ff723aa81..3bde903a691716adb14dcd57e2ea7b8b868d2e37 100644
--- a/components/metrics/metrics_provider.h
+++ b/components/metrics/metrics_provider.h
@@ -70,13 +70,20 @@ class MetricsProvider {
   virtual void ProvideGeneralMetrics(
       ChromeUserMetricsExtension* uma_proto);
 
-  // Called during collection to explicitly load histogram snapshots using a
-  // snapshot manager. PrepareDeltas() will have already been called and
-  // FinishDeltas() will be called later; calls to only PrepareDelta(), not
-  // PrepareDeltas (plural), should be made.
+  // Called during regular collection to explicitly load histogram snapshots
+  // using a snapshot manager. PrepareDeltas() will have already been called
+  // and FinishDeltas() will be called later; calls to only PrepareDelta(),
+  // not PrepareDeltas (plural), should be made.
   virtual void RecordHistogramSnapshots(
       base::HistogramSnapshotManager* snapshot_manager);
 
+  // Called during collection of "stability" metrics to explicitly load

[Ilya Sherman, 2016/05/04 06:48:08]

It's not actually "stability" that's relevant her.  Rather, this hook is for the
initial metrics log, associated with the previous session, right?  I'd clarify
the method name to mention "initial" rather than "stability", and not mention
"stability" in the docs.

[bcwhite, 2016/05/04 14:12:51]

Done.

+  // histogram snapshots using a snapshot manager. PrepareDeltas() will have
+  // already been called and FinishDeltas() will be called later; calls to
+  // only PrepareDelta(), not PrepareDeltas (plural), should be made.
+  virtual void RecordStabilityHistogramSnapshots(
+      base::HistogramSnapshotManager* snapshot_manager);
+
  private:
   DISALLOW_COPY_AND_ASSIGN(MetricsProvider);
 };