TestExpcatations: css3/filters/custom/composited/custom-filter-blend-modes.html is flak

BUG=282496
TBR=tkent@chromium.org

Review URL: https://codereview.chromium.org/23523007

git-svn-id: svn://svn.chromium.org/blink/trunk@157002 bbb929c8-8fbe-4397-9dbb-9b2b20218538