Web Animations CSS: Fix AnimatableColorImpl::toColor for transparent colors

Color's default ctor was changed from transparent to invalid in
https://codereview.chromium.org/23581008, and this call site wasn't
updated. This fixes it and adds a simple test.

BUG=257591

Review URL: https://codereview.chromium.org/25664002

git-svn-id: svn://svn.chromium.org/blink/trunk@158795 bbb929c8-8fbe-4397-9dbb-9b2b20218538