Add a binding test for Promise<T> syntax.

As chromium-side WebIDL parser supports Promise<T> syntax, this CL adds a
binding test for it.

BUG=421539

Review URL: https://codereview.chromium.org/666083003

git-svn-id: svn://svn.chromium.org/blink/trunk@184065 bbb929c8-8fbe-4397-9dbb-9b2b20218538