Add test for css transition using currentColor and box shadow

Add a new test that ensures that currentColor is handled currently
when used for a box shadow during a css transition animation.
This was about the most complex combination I could think of and
should help ensure that the currentColor refactoring work doesn't
break anything.

BUG=261854
R=leviw@chromium.org

Review URL: https://codereview.chromium.org/19795003

git-svn-id: svn://svn.chromium.org/blink/trunk@154584 bbb929c8-8fbe-4397-9dbb-9b2b20218538