Bug 1618011 part 4. Pass a BindingCallContext to dictionary Init() methods. r=peterv
Dictionaries always need a BindingCallContext, because they can throw
MSG_NOT_DICTIONARY from Init().
We allow non-binding callsites to pass JSContext*, in which case they will not
get quite-as-nice error reporting.
Differential Revision: https://phabricator.services.mozilla.com/D64885