Introduce isOverflowVisible()

Since it's illegal for overflowX() to be OVISIBLE and for
overflowY() to be something else (and vice versa), create a
helper function that ASSERTs this and removes the need to
write statements that look wrong to the uninitiated!

BUG=

Review URL: https://codereview.chromium.org/145003016

git-svn-id: svn://svn.chromium.org/blink/trunk@165975 bbb929c8-8fbe-4397-9dbb-9b2b20218538