Revert r154797: "Move isValid/isCurrentColor from Color to StyleColor"

r154797 caused a number of regressions and instead of trying to fix
them one at a time the change is reverted for now.

BUG=272678, 284357
R=leviw@chromium.org

Review URL: https://chromiumcodereview.appspot.com/23581008

git-svn-id: svn://svn.chromium.org/blink/trunk@157239 bbb929c8-8fbe-4397-9dbb-9b2b20218538