Web Animations: Add AnimatableLengthSize for animating border-radius

This patch adds the AnimatableLengthSize class used by the Web Animations
implementation for animating border-radius.
It adds better support fo AnimatableNumber for generating non-negative
CSS values.

The changes to the border-radius interpolation test uncover bugs in the
existing implementation: crbug.com/289792

BUG=257591

Review URL: https://chromiumcodereview.appspot.com/23503051

git-svn-id: svn://svn.chromium.org/blink/trunk@158513 bbb929c8-8fbe-4397-9dbb-9b2b20218538