Update failure expectation for fixed-body-background-positioned.html

This test is failing as text not image, so tweaking the expectation.

TBR=ccameron@chromium.org,pfeldman@chromium.org
BUG=310679

Review URL: https://codereview.chromium.org/34463009

git-svn-id: svn://svn.chromium.org/blink/trunk@160360 bbb929c8-8fbe-4397-9dbb-9b2b20218538