Web Animations: Support AnimatableNumbers creating non-negative calcs

AnimatableNumber assumed all calcs are allowed to be both positive
and negative. For some CSS properties this is not the case.

This patch adds support for AnimatableNumbers to restrict the range
of the calc values they generate.

BUG=257591

Review URL: https://chromiumcodereview.appspot.com/23440023

git-svn-id: svn://svn.chromium.org/blink/trunk@157591 bbb929c8-8fbe-4397-9dbb-9b2b20218538