Implement HTMLMarqueeElement.{scrollAmount,scrollDelay,loop} in Blink-in-JS

This CL removes C++ implementation for HTMLMarqueeElement.{scrollAmount,scrollDelay,loop}.

TEST=fast/dom/marquee-element.html
BUG=341031

Review URL: https://codereview.chromium.org/386343003

git-svn-id: svn://svn.chromium.org/blink/trunk@178146 bbb929c8-8fbe-4397-9dbb-9b2b20218538