Disable IDL methods returning a Dictionary

The bindings generator currently generates invalid code for this.

BUG=364025
R=kinuko@chromium.org

Review URL: https://codereview.chromium.org/267393002

git-svn-id: svn://svn.chromium.org/blink/trunk@173500 bbb929c8-8fbe-4397-9dbb-9b2b20218538