[trivial] Remove RenderBox::MarginSign

This patch removes the unused MarginSign enum. I noticed this while
searching how we name enums in Blink as part of another patch.

Since I was in the area, I fixed the comment about margins as well.

R=esprehn

Review URL: https://codereview.chromium.org/548113002

git-svn-id: svn://svn.chromium.org/blink/trunk@181528 bbb929c8-8fbe-4397-9dbb-9b2b20218538