Fix fast/html/marquee-scrollamount.html

- To make the image result deterministic, changes the character color of
the marquee element to white.

- Fix a bug in convertHTMLLengthToCSSLength(). '100px' should be
converted to '100px'. Previously '100px' was converted to null.

BUG=341031

Review URL: https://codereview.chromium.org/636663004

git-svn-id: svn://svn.chromium.org/blink/trunk@183325 bbb929c8-8fbe-4397-9dbb-9b2b20218538