Cleanup the way we compute the darker / lighter colors for inset / outset borders.
Cleanup the way we compute the darker / lighter colors for inset / outset borders.

Just so we have a single way to define the coefficients that we use to darken /
enlighten a color for these kinds of borders.

servo/webrender,servo/webrender,servo/webrender,servo/webrender,servo/webrender,servo/webrender