AX: CrashTracer: [USER] 296 crashes in WebProcess at com.apple.WebCore: WebCore:...

AX: CrashTracer: [USER] 296 crashes in WebProcess at com.apple.WebCore: WebCore::AccessibilityScrollbar::document const + 29
https://bugs.webkit.org/show_bug.cgi?id=69936

AX Scrollbars have a weak pointer to their parent. They need to become AccessibilityMockObjects, so that they can
participate in the detachFromParent() methods that happens when those parents go away.
Could not reproduce the crash, but the backtrace is unequivocal.

Reviewed by Darin Adler.

* accessibility/AccessibilityScrollView.cpp:
(WebCore::AccessibilityScrollView::removeChildScrollbar):
* accessibility/AccessibilityScrollbar.cpp:
(WebCore::AccessibilityScrollbar::AccessibilityScrollbar):
* accessibility/AccessibilityScrollbar.h:
(WebCore::AccessibilityScrollbar::scrollbar):
(WebCore::AccessibilityScrollbar::isAccessibilityScrollbar):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@97303 268f45cc-cd09-0410-ab3c-d52691b4dbfc