Fix getComputedStyle() for border-radius using CSS calc with percentages

Computing the border radius for elements currently neglects calcs that
contain percentages. The computed value assumes that 100% is 0px and
produces erroneous results.

This patch makes calculated border radii layout dependent and uses the
correct border box size to resolve the calc into an absolute length as
per spec: http://dev.w3.org/csswg/css-backgrounds/#the-border-radius

BUG=289090

Review URL: https://chromiumcodereview.appspot.com/23454023

git-svn-id: svn://svn.chromium.org/blink/trunk@157565 bbb929c8-8fbe-4397-9dbb-9b2b20218538