IDL: Fix overload resolution for dictionary types

Overload resolution only handled the "Dictionary" type correctly, not
actual declared dictionary types.

BUG=321462

Review URL: https://codereview.chromium.org/474813004

git-svn-id: svn://svn.chromium.org/blink/trunk@180527 bbb929c8-8fbe-4397-9dbb-9b2b20218538