AX: Support aria-posinset/setsize

AX: Support aria-posinset/setsize
https://bugs.webkit.org/show_bug.cgi?id=109725

Reviewed by Tim Horton.

Source/WebCore:

Expose aria-setsize and aria-posinset through accessibility.
Right now only the Mac platform exposes them.

Test: platform/mac/accessibility/aria-setsize-posinset.html

* accessibility/AccessibilityObject.cpp:
(WebCore::AccessibilityObject::supportsARIASetSize):
(WebCore):
(WebCore::AccessibilityObject::supportsARIAPosInset):
(WebCore::AccessibilityObject::ariaSetSize):
(WebCore::AccessibilityObject::ariaPosInset):
* accessibility/AccessibilityObject.h:
(AccessibilityObject):
* accessibility/mac/WebAccessibilityObjectWrapper.mm:
(-[WebAccessibilityObjectWrapper additionalAccessibilityAttributeNames]):
(-[WebAccessibilityObjectWrapper accessibilityAttributeValue:]):
* html/HTMLAttributeNames.in:

LayoutTests:

* platform/mac/accessibility/aria-setsize-posinset-expected.txt: Added.
* platform/mac/accessibility/aria-setsize-posinset.html: Added.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@144858 268f45cc-cd09-0410-ab3c-d52691b4dbfc