Element::attributeChanged shouldn't do any work when attribute value didn't change

Element::attributeChanged shouldn't do any work when attribute value didn't change
https://bugs.webkit.org/show_bug.cgi?id=129467

Reviewed by Geoffrey Garen.

Exit early in childrenChanged when the attribute value didn't change.

* dom/Attr.cpp:
(WebCore::Attr::setValue):
(WebCore::Attr::childrenChanged):
* dom/Element.cpp:
(WebCore::Element::setAttributeInternal):
(WebCore::Element::attributeChanged):
(WebCore::Element::parserSetAttributes):
(WebCore::Element::removeAttributeInternal):
(WebCore::Element::didAddAttribute):
(WebCore::Element::didModifyAttribute):
(WebCore::Element::didRemoveAttribute):
(WebCore::Element::cloneAttributesFromElement):
* dom/Element.h:
* dom/StyledElement.cpp:
(WebCore::StyledElement::attributeChanged):
* dom/StyledElement.h:
* html/HTMLInputElement.cpp:
(WebCore::HTMLInputElement::updateType):
* mathml/MathMLElement.cpp:
(WebCore::MathMLElement::attributeChanged):
* mathml/MathMLElement.h:
* mathml/MathMLSelectElement.cpp:
(WebCore::MathMLSelectElement::attributeChanged):
* mathml/MathMLSelectElement.h:
* svg/SVGElement.cpp:
(WebCore::SVGElement::attributeChanged):
* svg/SVGElement.h:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@164856 268f45cc-cd09-0410-ab3c-d52691b4dbfc