Bug 817194.  Make WebIDL dictionary types sanely initialized by default; add a new Fast subclass to handle cases when we can skip the initialization safely.  r=khuey
Backed out changeset 8ecb68f17618 (bug 909129) for causing bug 924959
Bug 817194 followup because someone added another *Initializer use.  r=bustage
Backed out changeset ccedb8b1cefc (bug 817194)
Backed out changeset 8fa0cbe4a54d (bug 817194) for build failures