IDL: Support callback function types in overload resolution

The specification has a "if Callable(V) is true, ..." case for (among
other things) callback function types, which is implemented here, only
for callback function types.

BUG=293561

Review URL: https://codereview.chromium.org/715353002

git-svn-id: svn://svn.chromium.org/blink/trunk@185230 bbb929c8-8fbe-4397-9dbb-9b2b20218538