AX: iOS: VoiceOver gets hung on some websites consistently.
https://bugs.webkit.org/show_bug.cgi?id=139331

Reviewed by Mario Sanchez Prada.

iFrames are attachments on iOS, but they do not have attachment views. As a result,
WebCore would return incorrect information for the element count and index of children elements.

No tests. Bug only manifests itself when iOS accessibility frameworks call into WebCore.

* accessibility/AccessibilityMockObject.h:
(WebCore::AccessibilityMockObject::isDetachedFromParent):
* accessibility/AccessibilityObject.h:
(WebCore::AccessibilityObject::isDetachedFromParent):
* accessibility/ios/WebAccessibilityObjectWrapperIOS.mm:
(-[WebAccessibilityObjectWrapper accessibilityElementCount]):
(-[WebAccessibilityObjectWrapper accessibilityElementAtIndex:]):
(-[WebAccessibilityObjectWrapper indexOfAccessibilityElement:]):
(-[WebAccessibilityObjectWrapper accessibilityContainer]):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@176948 268f45cc-cd09-0410-ab3c-d52691b4dbfc