Bug 1618011 part 5. Use a BindingCallContext for Web IDL callback guts. r=peterv

We basically need this because some of the type conversions those guts need to
do may need a BindingCallContext.

We could probably do more optimization here to only generate the
BindingCallContext bits if we will really need them, more or less based on our
return type. But for now that doesn't seem worthwhile.

Differential Revision: https://phabricator.services.mozilla.com/D64886