Unreviewed gardening. Remove a stale 10.6 baseline for compositing/fixed-body-background-positioned.html.

TBR=

Review URL: https://codereview.chromium.org/18733009

git-svn-id: svn://svn.chromium.org/blink/trunk@154166 bbb929c8-8fbe-4397-9dbb-9b2b20218538