Fix the flakiness of fast/html/marquee-scrollamount.html

The reason of the flakiness is that the test fails to set the text color to white.
This CL rewrites the test to set the color correctly.

BUG=421283

Review URL: https://codereview.chromium.org/634143002

git-svn-id: svn://svn.chromium.org/blink/trunk@183384 bbb929c8-8fbe-4397-9dbb-9b2b20218538