Makes use of EngineGeolocationFeature weak_ptr threadsafe.

blimp/engine/feature/geolocation/engine_geolocation_feature.cc

Index: blimp/engine/feature/geolocation/engine_geolocation_feature.cc
diff --git a/blimp/engine/feature/geolocation/engine_geolocation_feature.cc b/blimp/engine/feature/geolocation/engine_geolocation_feature.cc
index 3e8551db1b6b5b51e3e56513db08cc8ea7af1d8f..e63096fcda772a6202e5ba331c449eec074e6022 100644
--- a/blimp/engine/feature/geolocation/engine_geolocation_feature.cc
+++ b/blimp/engine/feature/geolocation/engine_geolocation_feature.cc
@@ -27,17 +27,20 @@ class BlimpGeolocationDelegate : public device::GeolocationDelegate {
   explicit BlimpGeolocationDelegate(
       base::WeakPtr<BlimpLocationProvider::Delegate> feature_delegate) {
     feature_delegate_ = feature_delegate;
+    feature_task_runner_ = base::ThreadTaskRunnerHandle::Get();

[Kevin M, 2016/09/09 17:07:05]

Do we need to store this as a field? Can we just get the handle in
OverrideSystemLocationProvider?

[CJ, 2016/09/09 19:52:41]

I don't believe so, since OverrideSystemLocationProvider is called on the
Geolocation Thread, not the Blimp thread. We want the Blimp thread in this case.

   }
 
   bool UseNetworkLocationProviders() final { return false; }
 
   std::unique_ptr<device::LocationProvider> OverrideSystemLocationProvider()
       final {
-    return base::MakeUnique<BlimpLocationProvider>(feature_delegate_);
+    return base::MakeUnique<BlimpLocationProvider>(feature_delegate_,
+                                                   feature_task_runner_);
   }
 
  private:
   base::WeakPtr<BlimpLocationProvider::Delegate> feature_delegate_;
+  scoped_refptr<base::SingleThreadTaskRunner> feature_task_runner_;
 
   DISALLOW_COPY_AND_ASSIGN(BlimpGeolocationDelegate);
 };
@@ -152,11 +155,11 @@ void EngineGeolocationFeature::OnPermissionGranted() {
 }
 
 void EngineGeolocationFeature::SetUpdateCallback(
+    scoped_refptr<base::SingleThreadTaskRunner> caller_task_runner,
     const GeopositionReceivedCallback& callback) {
   geoposition_received_callback_ = callback;
-
   // Set |callback_task_runner_| to run on the current thread.

[Kevin M, 2016/09/09 17:07:05]

This doesn't make sense?

[CJ, 2016/09/09 19:52:41]

Done.

-  callback_task_runner_ = base::ThreadTaskRunnerHandle::Get();
+  callback_task_runner_ = caller_task_runner;
 }
 
 }  // namespace engine