Web Animations CSS: Negative values of height and width are illegal

No change in behavior, just document the fact that height and width
can't be negative for consistency with the rest of this code.

[1] http://www.w3.org/TR/CSS2/visudet.html#the-height-property
[2] http://www.w3.org/TR/CSS2/visudet.html#the-width-property

BUG=257591

Review URL: https://codereview.chromium.org/26965002

git-svn-id: svn://svn.chromium.org/blink/trunk@159451 bbb929c8-8fbe-4397-9dbb-9b2b20218538