Unofficial gardening. Mark new createImageBitmap test as Slow. Rebaseline vertical-scroll-composited on XP.

BUG=166658
TBR=eseidel

Review URL: https://codereview.chromium.org/18938005

git-svn-id: svn://svn.chromium.org/blink/trunk@154189 bbb929c8-8fbe-4397-9dbb-9b2b20218538