Add MetricsProvider::OnAppEnterBackground callback.

components/metrics/metrics_service.cc

 // 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.
 
 //------------------------------------------------------------------------------
 // Description of the life cycle of a instance of MetricsService.
 //
 //  OVERVIEW
 //
 // A MetricsService instance is typically created at application startup.  It is
@@ skipping 451 matching lines @@
   LogCleanShutdown();
   RecordBooleanPrefValue(prefs::kStabilitySessionEndCompleted, true);
 }
 
 #if defined(OS_ANDROID) || defined(OS_IOS)
 void MetricsService::OnAppEnterBackground() {
   scheduler_->Stop();
 
   MarkAppCleanShutdownAndCommit(&clean_exit_beacon_, local_state_);
 
+  // Give providers a chance to persist histograms as part of being
+  // backgrounded.
+  for (MetricsProvider* provider : metrics_providers_)
+    provider->OnAppEnterBackground();
+
   // At this point, there's no way of knowing when the process will be
   // killed, so this has to be treated similar to a shutdown, closing and
   // persisting all logs. Unlinke a shutdown, the state is primed to be ready
   // to continue logging and uploading if the process does return.
   if (recording_active() && state_ >= SENDING_LOGS) {
     PushPendingLogsToPersistentStorage();
     // Persisting logs closes the current log, so start recording a new log
     // immediately to capture any background work that might be done before the
     // process is killed.
     OpenNewLog();
@@ skipping 692 matching lines @@
                  base::Time::Now().ToTimeT());
 }
 
 void MetricsService::SkipAndDiscardUpload() {
   log_manager_.DiscardStagedLog();
   scheduler_->UploadCancelled();
   log_upload_in_progress_ = false;
 }
 
 }  // namespace metrics