Mark fast/frames/remove-frame-during-load-event.html as flaky

BUG=361133
TBR=jianli@chromium.org

Review URL: https://codereview.chromium.org/228583003

git-svn-id: svn://svn.chromium.org/blink/trunk@171061 bbb929c8-8fbe-4397-9dbb-9b2b20218538