Mark heap-snapshot-with-detached-dom-tree.html as flaky.

TBR=yurys@chromium.org
BUG=312925

Review URL: https://codereview.chromium.org/51323002

git-svn-id: svn://svn.chromium.org/blink/trunk@160858 bbb929c8-8fbe-4397-9dbb-9b2b20218538