Remove dead code for webkitvisibilitychange in Document

There's no attribute EventHandler onwebkitvisibilitychange in the IDL,
so the DEFINE_ATTRIBUTE_EVENT_LISTENER would just output dead code.

BUG=none

Review URL: https://codereview.chromium.org/35693008

git-svn-id: svn://svn.chromium.org/blink/trunk@160346 bbb929c8-8fbe-4397-9dbb-9b2b20218538