Rewrite BorderImageLength to not use the Relative length type

Remove the last remaining Relative length and instead store a Length
_and_ a double value along with a type tag, a pattern also used by
GridLength in the Flexbox code. While it increases memory usage
slightly, getting rid of the Relative length type is deemed more
important.

BUG=259107

Review URL: https://codereview.chromium.org/55833002

git-svn-id: svn://svn.chromium.org/blink/trunk@161802 bbb929c8-8fbe-4397-9dbb-9b2b20218538