Web Animations CSS: Fix handling of background-color: initial

This patch fixes our handling of background-color: initial. We currently
treat all invalid colors as currentColor, but this means transparent for
background-color instead. This would lead to strange transitions when we
changed background-color from e.g. initial to transparent, which doesn't
actually change the computed style.

BUG=328305

Review URL: https://codereview.chromium.org/101833005

git-svn-id: svn://svn.chromium.org/blink/trunk@163938 bbb929c8-8fbe-4397-9dbb-9b2b20218538