Fix transitions/cross-fade-background-image.html

This test started failing after codereview.chromium.org/716963002. The
change to transitions/cross-fade-border-image.html in the linked patch
is similar. Since the tryservers don't have numbers in their paths we
didn't notice this test needed updating.

BUG=432566

Review URL: https://codereview.chromium.org/722753002

git-svn-id: svn://svn.chromium.org/blink/trunk@185231 bbb929c8-8fbe-4397-9dbb-9b2b20218538