Web Animations CSS: padding can't be negative

The padding properties (padding-left -right -top -bottom) can't
be negative [1]. Make Web Animations CSS follow the spec: use
animatableValueToLength() with option NonNegativeValues to set
these properties.

[1] http://dev.w3.org/csswg/css-box/#the-padding

BUG=257591

Review URL: https://codereview.chromium.org/26225006

git-svn-id: svn://svn.chromium.org/blink/trunk@159411 bbb929c8-8fbe-4397-9dbb-9b2b20218538