Gardening: Mark timing-function.html flaky

BUG=248938
TBR=ajuma@chromium.org

Review URL: https://codereview.chromium.org/189463010

git-svn-id: svn://svn.chromium.org/blink/trunk@168756 bbb929c8-8fbe-4397-9dbb-9b2b20218538