[chromecast] ExternalMetrics has to destroy itself in file thread.

chromecast/browser/metrics/external_metrics.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 CHROMECAST_BROWSER_METRICS_EXTERNAL_METRICS_H_
 #define CHROMECAST_BROWSER_METRICS_EXTERNAL_METRICS_H_
 
 #include <string>
 
 #include "base/macros.h"
 #include "base/memory/weak_ptr.h"
 
 namespace metrics {
 class MetricSample;
 }  // namespace metrics
 
 namespace chromecast {
 namespace metrics {
 
 class CastStabilityMetricsProvider;
 
 // ExternalMetrics service allows processes outside of the Chromecast browser
 // process to upload metrics via reading/writing to a known shared file.
 class ExternalMetrics {
  public:
   explicit ExternalMetrics(CastStabilityMetricsProvider* stability_provider);
-  ~ExternalMetrics();
 
   // Begins external data collection. Calls to RecordAction originate in the
   // File thread but are executed in the UI thread.
   void Start();
 
+  // Destroys itself in appropriate thread.
+  void StopAndDestroy();
+
  private:
+  friend class base::DeleteHelper<ExternalMetrics>;
+
+  ~ExternalMetrics();
+
   // The max length of a message (name-value pair, plus header)
   static const int kMetricsMessageMaxLength = 1024;  // be generous
 
   // Records an external crash of the given string description.
   void RecordCrash(const std::string& crash_kind);
 
   // Records a sparse histogram. |sample| is expected to be a sparse histogram.
   void RecordSparseHistogram(const ::metrics::MetricSample& sample);
 
   // Collects external events from metrics log file.  This is run at periodic
@@ skipping 16 matching lines @@
 
   base::WeakPtrFactory<ExternalMetrics> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(ExternalMetrics);
 };
 
 }  // namespace metrics
 }  // namespace chromecast
 
 #endif  // CHROMECAST_BROWSER_METRICS_EXTERNAL_METRICS_H_