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..20b73669f43e3a7d0ca38168cf211c890c8e9d92 100644
--- a/blimp/engine/feature/geolocation/blimp_location_provider.h
+++ b/blimp/engine/feature/geolocation/blimp_location_provider.h
@@ -17,11 +17,11 @@ namespace engine {
 class BlimpLocationProvider : public device::LocationProvider {
  public:
   // A delegate that implements a subset of LocationProvider's functions.
+  // All methods will be executed on |delegate_task_runner_|.
   class Delegate {
    public:
     using GeopositionReceivedCallback =
         base::Callback<void(const device::Geoposition&)>;
-
     virtual ~Delegate() {}
 
     virtual void RequestAccuracy(
@@ -31,7 +31,9 @@ class BlimpLocationProvider : public device::LocationProvider {
         const GeopositionReceivedCallback& callback) = 0;
   };
 
-  explicit BlimpLocationProvider(base::WeakPtr<Delegate> delegate);
+  BlimpLocationProvider(
+      base::WeakPtr<Delegate> delegate,
+      scoped_refptr<base::SequencedTaskRunner> delegate_task_runner);
   ~BlimpLocationProvider() override;
 
   // device::LocationProvider implementation.
@@ -46,11 +48,14 @@ class BlimpLocationProvider : public device::LocationProvider {
   // This delegate handles a subset of the LocationProvider functionality.
   base::WeakPtr<Delegate> delegate_;
 
+  scoped_refptr<base::SequencedTaskRunner> delegate_task_runner_;
   device::Geoposition cached_position_;
 
   // True if a successful StartProvider call has occured.
   bool is_started_;
 
+  base::WeakPtrFactory<BlimpLocationProvider> weak_factory_;
+
   DISALLOW_COPY_AND_ASSIGN(BlimpLocationProvider);
 };