Remove Document::idAttributeName().

Remove Document::idAttributeName().
<https://webkit.org/b/129663>

Reviewed by Ryosuke "DYEB" Niwa.

This abstraction is not actually used and causes unnecessary indirection
in some pretty hot code paths.

Replace it with hard-coded HTMLNames::idAttr instead which is a compile
time constant pointer. We can revisit this in the future if we wish to
implement support for custom id attributes.

* dom/Attr.cpp:
(WebCore::Attr::isId):
* dom/Document.cpp:
(WebCore::Document::Document):
* dom/Document.h:
* dom/Element.cpp:
(WebCore::Element::attributeChanged):
(WebCore::Element::willModifyAttribute):
* dom/Element.h:
(WebCore::Element::getIdAttribute):
(WebCore::Element::getNameAttribute):
(WebCore::Element::setIdAttribute):
* html/HTMLElement.cpp:
(WebCore::HTMLElement::parseAttribute):
* html/HTMLFrameElementBase.cpp:
(WebCore::HTMLFrameElementBase::parseAttribute):
* html/HTMLMapElement.cpp:
(WebCore::HTMLMapElement::parseAttribute):
* svg/SVGElement.cpp:
(WebCore::SVGElement::attributeChanged):
(WebCore::SVGElement::isKnownAttribute):
(WebCore::SVGElement::svgAttributeChanged):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@165045 268f45cc-cd09-0410-ab3c-d52691b4dbfc