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

device/bluetooth/bluetooth_adapter.cc

Index: device/bluetooth/bluetooth_adapter.cc
diff --git a/device/bluetooth/bluetooth_adapter.cc b/device/bluetooth/bluetooth_adapter.cc
index 14679d62c5149fb19820fd9f9ff367e554a5b545..23fde08cc578ee1baabf0b16bbb986c47e36bb04 100644
--- a/device/bluetooth/bluetooth_adapter.cc
+++ b/device/bluetooth/bluetooth_adapter.cc
@@ -34,6 +34,16 @@ void BluetoothAdapter::Shutdown() {
 }
 #endif
 
+void BluetoothAdapter::AddObserver(BluetoothAdapter::Observer* observer) {
+  DCHECK(observer);
+  observers_.AddObserver(observer);
+}
+
+void BluetoothAdapter::RemoveObserver(BluetoothAdapter::Observer* observer) {
+  DCHECK(observer);
+  observers_.RemoveObserver(observer);
+}
+
 void BluetoothAdapter::StartDiscoverySession(
     const DiscoverySessionCallback& callback,
     const ErrorCallback& error_callback) {