Mark following tests flaky.

* http/tests/xmlhttprequest/workers/close.html
* compositing/overflow/do-not-crash-use-after-free-update-widget-positions.html
* virtual/gpu/compositedscrolling/overflow/do-not-crash-use-after-free-update-widget-positions.html

BUG=409534,409535
TBR=hclam@chromium.org

Review URL: https://codereview.chromium.org/528723002

git-svn-id: svn://svn.chromium.org/blink/trunk@181156 bbb929c8-8fbe-4397-9dbb-9b2b20218538