IDL compiler: Methods with basic type return type

Mostly already implemented by type handling.

BUG=239771
NOTRY=true

Review URL: https://codereview.chromium.org/47983002

git-svn-id: svn://svn.chromium.org/blink/trunk@160694 bbb929c8-8fbe-4397-9dbb-9b2b20218538