Add MetricsProvider::OnAppEnterBackground callback.

components/metrics/metrics_provider.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef COMPONENTS_METRICS_METRICS_PROVIDER_H_
 #define COMPONENTS_METRICS_METRICS_PROVIDER_H_
 
 #include "base/macros.h"
 
 namespace base {
@@ skipping 18 matching lines @@
 
   // Called when a new MetricsLog is created.
   virtual void OnDidCreateMetricsLog();
 
   // Called when metrics recording has been enabled.
   virtual void OnRecordingEnabled();
 
   // Called when metrics recording has been disabled.
   virtual void OnRecordingDisabled();
 
+  // Called when the application is going into background mode, on platforms
+  // where applications may be killed when going into the background (Android,
+  // iOS). Providers that buffer histogram data in memory should persist
+  // histograms in this callback or in the subsequent Provide* callbacks, as the
+  // application may be killed without further notification after this callback.

[Alexei Svitkine (slow), 2016/07/27 17:41:11]

This seems OK for now, but note that we do have plans to persist metrics even in
the case of a crash with bcwhite@'s persistent metrics work.

[Bryan McQuade, 2016/07/27 18:25:15]

Yeah - I think any subsystem that does buffering is going to continue to have to
accept data loss due to crashes. That's an acceptable tradeoff for us - there
are certain metrics that we only want to log once we've collected enough
information, and we accept that a low loss rate due to browser process crashes
is acceptable.

But we can at least flush those metrics when the app goes into the background,
using this callback. So I think this will continue to be beneficial for us even
with bcwhite's persistent metrics.

+  virtual void OnAppEnterBackground();
+
   // Provides additional metrics into the system profile.
   virtual void ProvideSystemProfileMetrics(
       SystemProfileProto* system_profile_proto);
 
   // Called once at startup to see whether this provider has critical stability
   // events to share in an initial stability log.
   // Returning true can trigger ProvideInitialStabilityMetrics and
   // ProvideStabilityMetrics on all other registered metrics providers.
   // Default implementation always returns false.
   virtual bool HasInitialStabilityMetrics();
@@ skipping 39 matching lines @@
   virtual void RecordInitialHistogramSnapshots(
       base::HistogramSnapshotManager* snapshot_manager);
 
  private:
   DISALLOW_COPY_AND_ASSIGN(MetricsProvider);
 };
 
 }  // namespace metrics
 
 #endif  // COMPONENTS_METRICS_METRICS_PROVIDER_H_