Web Animations CSS: -webkit-column-gap can't be negative

Follow up https://src.chromium.org/viewvc/blink?view=rev&revision=158780,
add a comment to the -webkit-column-gap interpolation test about the gap
value: it can't be negative [1].

[1] http://dev.w3.org/csswg/css-multicol/#column-gap

BUG=257591

Review URL: https://codereview.chromium.org/26541004

git-svn-id: svn://svn.chromium.org/blink/trunk@159225 bbb929c8-8fbe-4397-9dbb-9b2b20218538