bluetooth: Refactor (Add|Remove)Observer impls into BluetoothAdapter.

device/bluetooth/bluetooth_adapter_win.h

Index: device/bluetooth/bluetooth_adapter_win.h
diff --git a/device/bluetooth/bluetooth_adapter_win.h b/device/bluetooth/bluetooth_adapter_win.h
index 2b086a2f9fb78e105562f04804fd0041d859f3b7..bf22f3db061e3a206792a2a3a9de20204a91f867 100644
--- a/device/bluetooth/bluetooth_adapter_win.h
+++ b/device/bluetooth/bluetooth_adapter_win.h
@@ -41,8 +41,6 @@ class DEVICE_BLUETOOTH_EXPORT BluetoothAdapterWin
       const InitCallback& init_callback);
 
   // BluetoothAdapter:
-  virtual void AddObserver(BluetoothAdapter::Observer* observer) override;
-  virtual void RemoveObserver(BluetoothAdapter::Observer* observer) override;
   virtual std::string GetAddress() const override;
   virtual std::string GetName() const override;
   virtual void SetName(const std::string& name,
@@ -152,9 +150,6 @@ class DEVICE_BLUETOOTH_EXPORT BluetoothAdapterWin
 
   base::ThreadChecker thread_checker_;
 
-  // List of observers interested in event notifications from us.
-  ObserverList<BluetoothAdapter::Observer> observers_;
-
   // NOTE: This should remain the last member so it'll be destroyed and
   // invalidate its weak pointers before any other members are destroyed.
   base::WeakPtrFactory<BluetoothAdapterWin> weak_ptr_factory_;