Don't ignore landmark and related AXObjects, because they should be exposed to
assistive technologies.

BUG=253027
TEST=LayoutTest/accessibility/not-ignore-landmark-roles.html
R=dmazzoni@chromium.org

Review URL: https://codereview.chromium.org/104583006

git-svn-id: svn://svn.chromium.org/blink/trunk@164498 bbb929c8-8fbe-4397-9dbb-9b2b20218538