AX: WebArea claims to have a subrole, but always returns nil.

AX: WebArea claims to have a subrole, but always returns nil.
https://bugs.webkit.org/show_bug.cgi?id=121658

Patch by Sam White <samuel_white@apple.com> on 2013-09-20
Reviewed by Chris Fleizach.

Source/WebCore:

No new tests, updated existing AXWebArea related test to reflect unused attribute removal.

* accessibility/mac/WebAccessibilityObjectWrapperMac.mm:
(-[WebAccessibilityObjectWrapper accessibilityAttributeNames]):

LayoutTests:

Updated test output to reflect removal of unused NSAccessibilitySubroleAttribute in AXWebArea.

* platform/mac/accessibility/document-attributes-expected.txt:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@156168 268f45cc-cd09-0410-ab3c-d52691b4dbfc