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..6d37a5229705644cdf757e70225ee9747a7d657f 100644
--- a/blimp/engine/feature/geolocation/blimp_location_provider.h
+++ b/blimp/engine/feature/geolocation/blimp_location_provider.h
@@ -14,9 +14,12 @@ namespace blimp {
 namespace engine {
 
 // Location provider for Blimp using the device's provider over the network.
+// All calls made from BlimpLocationProvider involving |delegate_| must be
+// posted using |delegate_task_runner_|.

[Wez, 2016/09/13 20:19:07]

nit: This is part of the Delegate contract, so you don't need to stipulate it
here - just mention it in the Delegate comment, or a comment on the constructor.

[CJ, 2016/09/13 23:34:02]

Done.

 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 =
@@ -31,7 +34,9 @@ class BlimpLocationProvider : public device::LocationProvider {
         const GeopositionReceivedCallback& callback) = 0;
   };
 
-  explicit BlimpLocationProvider(base::WeakPtr<Delegate> delegate);
+  BlimpLocationProvider(
+      base::WeakPtr<Delegate> delegate,
+      scoped_refptr<base::SingleThreadTaskRunner> delegate_task_runner);

[Wez, 2016/09/13 20:19:07]

nit: Strictly a SequencedTaskRunner is sufficient here, since that's the
requirement WeakPtr itself has

[CJ, 2016/09/13 23:34:02]

Are you saying remove the scoped_refptr part?

[Wez, 2016/09/16 00:40:16]

So, I'm saying you should accept a scoped_refptr<SequencedTaskRunner> rather
than <SingleThreadTaskRunner>, since strictly you only need the sequenced
property, and don't care if it's a dedicated thread.

[CJ, 2016/09/16 22:55:53]

Done.

   ~BlimpLocationProvider() override;
 
   // device::LocationProvider implementation.
@@ -46,6 +51,8 @@ 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.