Mark compositing/overflow/iframe-scroll-children.html as flaky

Since layer squashing was reenabled in r171962 the three versions of compositing/overflow/iframe-scroll-children.html have started ASSERTING on debug builds.

TBR=abarth@chromium.org
BUG=364991

Review URL: https://codereview.chromium.org/242803005

git-svn-id: svn://svn.chromium.org/blink/trunk@171976 bbb929c8-8fbe-4397-9dbb-9b2b20218538