Mark console/console-xml-document.html flaky on Linux.

BUG=406283
TBR=lushnikov@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/511843002

git-svn-id: svn://svn.chromium.org/blink/trunk@180970 bbb929c8-8fbe-4397-9dbb-9b2b20218538