IDL: Add test cases for union type return values

This is a follow up CL to r180360. Added cases are:
- GarbageCollected and WillBeGarbageCollected interfaces
- IDL Dictionary
- sequence<T> and T[]
- union type which has more than two members

Adding test cases found two bugs:
- IDL dictionaries should not be wrapped in Nullable
- IdlUnionType.includes_for_type should not use
  includes_for_type() as a function. includes_for_type
  is supposed to be used as a property.
This CL fixes the bugs.

Review URL: https://codereview.chromium.org/480783003

git-svn-id: svn://svn.chromium.org/blink/trunk@180617 bbb929c8-8fbe-4397-9dbb-9b2b20218538