Use is<>() / downcast<>() for ElementData subclasses
https://bugs.webkit.org/show_bug.cgi?id=137430

Reviewed by Benjamin Poulain.

Use is<>() / downcast<>() for ElementData subclasses.

No new tests, no behavior change.

* dom/Element.cpp:
(WebCore::Element::cloneAttributesFromElement):
(WebCore::Element::createUniqueElementData):
* dom/ElementData.cpp:
(WebCore::ElementData::destroy):
* dom/ElementData.h:
(WebCore::ElementData::length):
(WebCore::ElementData::attributeBase):
(WebCore::ElementData::presentationAttributeStyle):
(WebCore::ElementData::attributesIterator):
(WebCore::ElementData::findAttributeByName):
(isType):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@174329 268f45cc-cd09-0410-ab3c-d52691b4dbfc