Mark test createDocument-with-used-doctype.html as Slow instead of Pass/Timeout

BUG=68436
TBR=tkent

Review URL: https://codereview.chromium.org/20461002

git-svn-id: svn://svn.chromium.org/blink/trunk@154933 bbb929c8-8fbe-4397-9dbb-9b2b20218538