IDL: Support callback interface in overload resolution

Per the spec[1] we can treat an object as a callback interface
if an effective overload set contains the callback interface.

[1] http://heycam.github.io/webidl/#es-overloads

BUG=629068

Review-Url: https://codereview.chromium.org/2175463002
Cr-Commit-Position: refs/heads/master@{#407077}
3 files changed