https://bugs.webkit.org/show_bug.cgi?id=28841

Reviewed by Beth Dakin.

WebCore: 

Allow certain ARIA roles to change value with AXIncrement and AXDecrement. 
Consolidate increment/decrement/changeValue methods. 

Test: accessibility/aria-slider-value-change.html

* accessibility/AccessibilityObject.cpp:
* accessibility/AccessibilityRenderObject.cpp:
(WebCore::AccessibilityRenderObject::increment):
(WebCore::AccessibilityRenderObject::decrement):
(WebCore::AccessibilityRenderObject::changeValueByPercent):
(WebCore::AccessibilityRenderObject::setValue):
(WebCore::AccessibilityRenderObject::canSetFocusAttribute):
* accessibility/AccessibilityRenderObject.h:
* accessibility/AccessibilitySlider.cpp:
* accessibility/AccessibilitySlider.h:
* dom/Element.cpp:
(WebCore::Element::updateAfterAttributeChanged):

LayoutTests: 

* accessibility/aria-slider-value-change-expected.txt: Added.
* accessibility/aria-slider-value-change.html: Added.



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@48405 268f45cc-cd09-0410-ab3c-d52691b4dbfc