Added displayId to Gamepad

ppapi/shared_impl/ppb_gamepad_shared.h

Index: ppapi/shared_impl/ppb_gamepad_shared.h
diff --git a/ppapi/shared_impl/ppb_gamepad_shared.h b/ppapi/shared_impl/ppb_gamepad_shared.h
index 8787320ef9929a95d6f10c4109814b2528006b68..3a3d6802c09364d3a9f845d65e7ec6f747e63c62 100644
--- a/ppapi/shared_impl/ppb_gamepad_shared.h
+++ b/ppapi/shared_impl/ppb_gamepad_shared.h
@@ -91,6 +91,9 @@ struct WebKitGamepad {
   WebKitGamepadPose pose;
 
   WebKitGamepadHand hand;
+
+  // ID of the VRDisplay this gamepad is associated with, if any.
+  unsigned display_id;
 };
 
 // This must match the definition of blink::Gamepads. The GamepadHost unit