Makes use of EngineGeolocationFeature weak_ptr threadsafe.

blimp/engine/feature/geolocation/blimp_location_provider.h

Index: blimp/engine/feature/geolocation/blimp_location_provider.h
diff --git a/blimp/engine/feature/geolocation/blimp_location_provider.h b/blimp/engine/feature/geolocation/blimp_location_provider.h
index 39372da41c5dda1edc1d186653844ac6a5bce41c..314994bb11224a88c6d7677b94d1e1be3a9892dd 100644
--- a/blimp/engine/feature/geolocation/blimp_location_provider.h
+++ b/blimp/engine/feature/geolocation/blimp_location_provider.h
@@ -28,10 +28,13 @@ class BlimpLocationProvider : public device::LocationProvider {
         GeolocationSetInterestLevelMessage::Level level) = 0;
     virtual void OnPermissionGranted() = 0;
     virtual void SetUpdateCallback(
+        scoped_refptr<base::SingleThreadTaskRunner> caller_task_runner,
         const GeopositionReceivedCallback& callback) = 0;
   };
 
-  explicit BlimpLocationProvider(base::WeakPtr<Delegate> delegate);
+  BlimpLocationProvider(
+      base::WeakPtr<Delegate> delegate,
+      scoped_refptr<base::SingleThreadTaskRunner> delegate_task_runner);
   ~BlimpLocationProvider() override;
 
   // device::LocationProvider implementation.
@@ -46,6 +49,8 @@ class BlimpLocationProvider : public device::LocationProvider {
   // This delegate handles a subset of the LocationProvider functionality.
   base::WeakPtr<Delegate> delegate_;
 
+  scoped_refptr<base::SingleThreadTaskRunner> delegate_task_runner_;
+
   device::Geoposition cached_position_;
 
   // True if a successful StartProvider call has occured.