Use is<>() / downcast<>() for HTMLCollection subclasses
https://bugs.webkit.org/show_bug.cgi?id=138541

Reviewed by Sam Weinig.

Use is<>() / downcast<>() for HTMLCollection subclasses for
safety and consistency with the rest of the code base.

No new tests, no behavior change.

* bindings/gobject/WebKitDOMPrivate.cpp:
(WebKit::wrap):
* html/HTMLAllCollection.h:
* html/HTMLCollection.cpp:
(WebCore::isMatchingHTMLElement):
(WebCore::isMatchingElement):
* html/HTMLCollection.h:
* html/HTMLFormControlsCollection.h:
* html/HTMLNameCollection.h:
* html/HTMLOptionsCollection.h:
* html/HTMLSelectElement.cpp:
(WebCore::HTMLSelectElement::options):
* html/HTMLTableRowsCollection.h:


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@175791 268f45cc-cd09-0410-ab3c-d52691b4dbfc