Simplify an error condition in requestFullScreenForElement()

Use the existing getter for the top element and do the ancestor test
in a more direct manner.

BUG=383813
R=falken@chromium.org

Review URL: https://codereview.chromium.org/392913002

git-svn-id: svn://svn.chromium.org/blink/trunk@178360 bbb929c8-8fbe-4397-9dbb-9b2b20218538