Implement isEnabledFormControl() for SliderThumbElement and SpinButtonElement in terms of disabled()
https://bugs.webkit.org/show_bug.cgi?id=112993

Reviewed by Kent Tamura.

This is an intermediate step for refactoring Node::disabled and
Element::isEnabledFormControl. All classes that override
isEnabledFormControl just return !disabled(), except for
SliderThumbElement and SpinButtonElement. This patch changes those two
classes to do so, which shows that the two functions are redundant and
easy to refactor into a single function in a subsequent patch.

No new tests, there should be no behavior change.

* html/shadow/SliderThumbElement.cpp:
(WebCore::SliderThumbElement::disabled):
(WebCore):
(WebCore::SliderThumbElement::isEnabledFormControl):
* html/shadow/SliderThumbElement.h:
(SliderThumbElement):
* html/shadow/SpinButtonElement.h: The check of shadowHost() is
necessary because it can be detached by JavaScript that runs during
event handling (see r76566)


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@146638 268f45cc-cd09-0410-ab3c-d52691b4dbfc