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

device/bluetooth/bluetooth_adapter.h

Index: device/bluetooth/bluetooth_adapter.h
diff --git a/device/bluetooth/bluetooth_adapter.h b/device/bluetooth/bluetooth_adapter.h
index e2ebf48eb3a974beb6f7094384fb1743282e3c5f..4841048a35c4a324298565c849dc3b9676cbc35a 100644
--- a/device/bluetooth/bluetooth_adapter.h
+++ b/device/bluetooth/bluetooth_adapter.h
@@ -215,9 +215,8 @@ class DEVICE_BLUETOOTH_EXPORT BluetoothAdapter
   // Adds and removes observers for events on this bluetooth adapter. If
   // monitoring multiple adapters, check the |adapter| parameter of observer
   // methods to determine which adapter is issuing the event.
-  virtual void AddObserver(BluetoothAdapter::Observer* observer) = 0;
-  virtual void RemoveObserver(
-      BluetoothAdapter::Observer* observer) = 0;
+  virtual void AddObserver(BluetoothAdapter::Observer* observer);
+  virtual void RemoveObserver(BluetoothAdapter::Observer* observer);
 
   // The address of this adapter. The address format is "XX:XX:XX:XX:XX:XX",
   // where each XX is a hexadecimal number.
@@ -454,6 +453,9 @@ class DEVICE_BLUETOOTH_EXPORT BluetoothAdapter
   void DiscoverySessionBecameInactive(
       BluetoothDiscoverySession* discovery_session);
 
+  // Observers of BluetoothAdapter, notified from implementation subclasses.
+  ObserverList<device::BluetoothAdapter::Observer> observers_;
+
   // Devices paired with, connected to, discovered by, or visible to the
   // adapter. The key is the Bluetooth address of the device and the value is
   // the BluetoothDevice object whose lifetime is managed by the adapter