bluetooth: add DisplayPinCode method

Our bluetoothd is patched to call DisplayPinCode on the agent when
pairing with Bluetooth 2.0 and older keyboards. Add this method to
the bluetooth_agent interface.

BUG=chromium-os:27101
TEST=none

Change-Id: I749a449b06a8673be93145dc292a18bb4424edbe
diff --git a/dbus/service_constants.h b/dbus/service_constants.h
index 4389e9e..557fd1e 100644
--- a/dbus/service_constants.h
+++ b/dbus/service_constants.h
@@ -727,6 +727,7 @@
 const char kRelease[] = "Release";
 const char kRequestPinCode[] = "RequestPinCode";
 const char kRequestPasskey[] = "RequestPasskey";
+const char kDisplayPinCode[] = "DisplayPinCode";
 const char kDisplayPasskey[] = "DisplayPasskey";
 const char kRequestConfirmation[] = "RequestConfirmation";
 const char kAuthorize[] = "Authorize";