Fixing test expectation that was set in r177785

Changing "Failure" to "ImageOnlyFailure" for layout test
svg/custom/svg-font-support.html on XP

BUG=242735
TBR=yutak@chromium.org,pdr@chromium.org

Review URL: https://codereview.chromium.org/380213005

git-svn-id: svn://svn.chromium.org/blink/trunk@177836 bbb929c8-8fbe-4397-9dbb-9b2b20218538