IDL compiler: Methods with interface type return type

V. simple one; interface type arguments are more involved, and I'll do
those in a separate CL.

BUG=239771
NOTRY=true

Review URL: https://codereview.chromium.org/49093007

git-svn-id: svn://svn.chromium.org/blink/trunk@160776 bbb929c8-8fbe-4397-9dbb-9b2b20218538