Broaden flaky expectations for virtual/gpu/compositedscrolling/overflow/overflow-positioning.html

BUG=247982
TBR=haraken

Review URL: https://codereview.chromium.org/20453002

git-svn-id: svn://svn.chromium.org/blink/trunk@154932 bbb929c8-8fbe-4397-9dbb-9b2b20218538