Makes use of EngineGeolocationFeature weak_ptr threadsafe.

blimp/engine/feature/geolocation/engine_geolocation_feature.h

Index: blimp/engine/feature/geolocation/engine_geolocation_feature.h
diff --git a/blimp/engine/feature/geolocation/engine_geolocation_feature.h b/blimp/engine/feature/geolocation/engine_geolocation_feature.h
index d6b1652cb724ad71da52d20ab55199992c476b8d..2b2765dc7e929b09f3a5b81d931159dd83253cfd 100644
--- a/blimp/engine/feature/geolocation/engine_geolocation_feature.h
+++ b/blimp/engine/feature/geolocation/engine_geolocation_feature.h
@@ -43,7 +43,9 @@ class EngineGeolocationFeature : public BlimpMessageProcessor,
   void RequestAccuracy(
       GeolocationSetInterestLevelMessage::Level level) override;
   void OnPermissionGranted() override;
-  void SetUpdateCallback(const GeopositionReceivedCallback& callback) override;
+  void SetUpdateCallback(
+      scoped_refptr<base::SingleThreadTaskRunner> caller_task_runner,
+      const GeopositionReceivedCallback& callback) override;
 
   std::unique_ptr<BlimpMessageProcessor> outgoing_message_processor_;
   GeopositionReceivedCallback geoposition_received_callback_;