Update tests to no longer use window.internals.

It isn't needed in Blink or in WebKit, so I've removed it upstream in
the CSSWG repo. This merges that change into Blink.

BUG=260357

Review URL: https://chromiumcodereview.appspot.com/20995005

git-svn-id: svn://svn.chromium.org/blink/trunk@155271 bbb929c8-8fbe-4397-9dbb-9b2b20218538