Refactor DOM Iterator next signature

Refactor DOM Iterator next signature
https://bugs.webkit.org/show_bug.cgi?id=154531

Reviewed by Myles C. Maxfield.

Covered by existing tests.

* Modules/fetch/FetchHeaders.cpp:
(WebCore::FetchHeaders::Iterator::next): Using Optional<KeyValuePair> to return iterator value.
* Modules/fetch/FetchHeaders.h:
* bindings/js/JSKeyValueIterator.h: Using Optional<KeyValuePair> as returned iterator value.
(WebCore::keyValueIteratorForEach):
(WebCore::JSKeyValueIterator<JSWrapper>::next):
* css/FontFaceSet.cpp:
(WebCore::FontFaceSet::Iterator::next): Using Optional<KeyValuePair> to return iterator value.
* css/FontFaceSet.h:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@196973 268f45cc-cd09-0410-ab3c-d52691b4dbfc