Makes use of EngineGeolocationFeature weak_ptr threadsafe.

blimp/engine/feature/geolocation/engine_geolocation_feature.h

 // Copyright 2016 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 BLIMP_ENGINE_FEATURE_GEOLOCATION_ENGINE_GEOLOCATION_FEATURE_H_
 #define BLIMP_ENGINE_FEATURE_GEOLOCATION_ENGINE_GEOLOCATION_FEATURE_H_
 
 #include <memory>
 
 #include "base/memory/weak_ptr.h"
@@ skipping 29 matching lines @@
   void NotifyCallback(const device::Geoposition& position);
 
   // BlimpLocationProvider::Delegate implementation.
   void RequestAccuracy(
       GeolocationSetInterestLevelMessage::Level level) override;
   void OnPermissionGranted() override;
   void SetUpdateCallback(const GeopositionReceivedCallback& callback) override;
 
   std::unique_ptr<BlimpMessageProcessor> outgoing_message_processor_;
   GeopositionReceivedCallback geoposition_received_callback_;
-  scoped_refptr<base::SingleThreadTaskRunner> callback_task_runner_;
   base::WeakPtrFactory<EngineGeolocationFeature> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(EngineGeolocationFeature);
 };
 
 }  // namespace engine
 }  // namespace blimp
 
 #endif  // BLIMP_ENGINE_FEATURE_GEOLOCATION_ENGINE_GEOLOCATION_FEATURE_H_