IDL: Use Nullable for union type return value

Before this CL, we use bool flags to indicate which value should be
returned. We can use Nullable<T> instead.

Review URL: https://codereview.chromium.org/466323002

git-svn-id: svn://svn.chromium.org/blink/trunk@180360 bbb929c8-8fbe-4397-9dbb-9b2b20218538