Web Animations: Support AnimatableNumber creating non-negative Lengths

This adds support to AnimatableNumber toLength to indicate whether the
value should be clamped to a non-negative range. This patch builds upon
AnimatableNumbers generating non-negative calcs by extending it to
Lengths.

BUG=257591

Review URL: https://chromiumcodereview.appspot.com/24168002

git-svn-id: svn://svn.chromium.org/blink/trunk@157879 bbb929c8-8fbe-4397-9dbb-9b2b20218538