Remove core/css from core/platform/DEPS

Also, add "mozilla" to the platform/DEPS to make CheckedInt.h happy.

R=schenney@chromium.org

Review URL: https://codereview.chromium.org/63173003

git-svn-id: svn://svn.chromium.org/blink/trunk@161507 bbb929c8-8fbe-4397-9dbb-9b2b20218538