IDL: Generic Dictionary support in union types

Even though we want to avoid using generic Dictionary in IDL
files, we need them in some cases. For example, WebCrypto
uses generic Dictionary because we don't have IDL dictionary
inheritance support. Add generic Dictionary support so that
such customers can use union types.

BUG=240176

Review URL: https://codereview.chromium.org/713403004

git-svn-id: svn://svn.chromium.org/blink/trunk@185189 bbb929c8-8fbe-4397-9dbb-9b2b20218538