Mark fast/html/marquee-scrollamount as flaky after r183245.

This test was updated as part of moving marquee to blink-in-js.
Looks like it's still a bit flaky.

TBR=jsbell@chromium.org, haraken@chromium.org
BUG=341031

Review URL: https://codereview.chromium.org/633583002

git-svn-id: svn://svn.chromium.org/blink/trunk@183276 bbb929c8-8fbe-4397-9dbb-9b2b20218538