Remove unused using declaration fom StyleColor.cpp

BUG=259121
R=leviw@chromium.org

Review URL: https://chromiumcodereview.appspot.com/19700010

git-svn-id: svn://svn.chromium.org/blink/trunk@154863 bbb929c8-8fbe-4397-9dbb-9b2b20218538