TestExpectation changes for virtual gpu deferred image tests

BUG=364946
TBR=danakj@chromium.org

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

Review URL: https://codereview.chromium.org/243633004

git-svn-id: svn://svn.chromium.org/blink/trunk@171978 bbb929c8-8fbe-4397-9dbb-9b2b20218538