Reflected attributes should be removed when 'null', 'undefined' or 'false' is specified

This is a follow-up CL to address arv's comment in https://codereview.chromium.org/386353002/#msg9.

Note that this CL will change existing behavior for reflected DOM attributes of HTMLMarqueeElement. Previously those reflected DOM attributes were not removed when 'null' or 'undefined' is specified. This was a bug. The new behavior implemented in this CL aligns with the spec and Firefox.

I don't have a concern about compatibility since this just affects reflected attributes of HTMLMarqueeElement.

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

Review URL: https://codereview.chromium.org/393543002

git-svn-id: svn://svn.chromium.org/blink/trunk@178076 bbb929c8-8fbe-4397-9dbb-9b2b20218538