Mark premultiplyapha-test.html as flaky on windows

BUG=321787
TEST=
TBR=kbr@chromium.org

Review URL: https://codereview.chromium.org/77763008

git-svn-id: svn://svn.chromium.org/blink/trunk@162413 bbb929c8-8fbe-4397-9dbb-9b2b20218538