Adjust compositing/layer-creation/fixed-position tests for FCM.

This makes the test easier to rebaseline when ForceCompositingMode is
enabled, removing the then-incorrect text from it.

R=enne@chromium.org
BUG=361729

Review URL: https://codereview.chromium.org/224883006

git-svn-id: svn://svn.chromium.org/blink/trunk@171188 bbb929c8-8fbe-4397-9dbb-9b2b20218538