Disable DumpAccessibilityTree tests which are broken by adding new elements in the Blink a11y tree

BUG=98976
R=dmazzoni@chromium.org

Review URL: https://codereview.chromium.org/196723006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256551 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256551