Gardening. Mark extra-transition.html as flaky failure in web-animations-css.

NOTRY=true
TBR=steveblock

Review URL: https://codereview.chromium.org/39993003

git-svn-id: svn://svn.chromium.org/blink/trunk@160443 bbb929c8-8fbe-4397-9dbb-9b2b20218538