Gardening: canvas-blending-pattern-over-gradient.html is flaky

BUG=312613
TBR=dglazkov

Review URL: https://codereview.chromium.org/50463002

git-svn-id: svn://svn.chromium.org/blink/trunk@160814 bbb929c8-8fbe-4397-9dbb-9b2b20218538