IDL: Introduce IdlTypeBase

IdlType and IdlUnionType have similar structures, so it might be good to have
the common superclass.

BUG=345503

Review URL: https://codereview.chromium.org/410103002

git-svn-id: svn://svn.chromium.org/blink/trunk@178884 bbb929c8-8fbe-4397-9dbb-9b2b20218538