[Runtime] Add the initialization for the struct members.

This patch is to add the initialization for the members in FieldIds.

CID=220063

BUG=XWALK-2928
omitting is not working because there is another user-defined ctor, to which I didn't pay attention in the beginning.
using '=default' forces deafult ctor generation. It should work.
http://en.cppreference.com/w/cpp/language/default_constructor