Elements retained by AccessibilityImageMapLink leak

Elements retained by AccessibilityImageMapLink leak
https://bugs.webkit.org/show_bug.cgi?id=120917

Reviewed by Chris Fleizach.

Merge https://chromium.googlesource.com/chromium/blink/+/df0bd288efe65e5e38425fbdcdfc68b5a2a79e42

Unfortunately, the test added by the Blink change doesn't reproduce a leak in WebKit.

* accessibility/AccessibilityImageMapLink.cpp:
(WebCore::AccessibilityImageMapLink::detachFromParent):
* accessibility/AccessibilityImageMapLink.h:
* accessibility/AccessibilityMockObject.h:
(WebCore::AccessibilityMockObject::detachFromParent):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@155234 268f45cc-cd09-0410-ab3c-d52691b4dbfc