Mark animations/compositor-start-event-timing.html as flaky

BUG=364640
TBR=dstockwell@chromium.org

Review URL: https://codereview.chromium.org/241473002

git-svn-id: svn://svn.chromium.org/blink/trunk@171883 bbb929c8-8fbe-4397-9dbb-9b2b20218538