IDL compiler: Callback interfaces as argument type (initial)

Plumbing is analogous implementation to [ImplementedAs] on types.
There are various places in the CG that check for callback interfaces;
this does the plumbing (is_callback_interface) and the simplest CG change.
Follow-ups will include other CG cases.

BUG=239771
R=haraken

Review URL: https://codereview.chromium.org/127103005

git-svn-id: svn://svn.chromium.org/blink/trunk@164730 bbb929c8-8fbe-4397-9dbb-9b2b20218538