binding: Adds a comment why SameSizeAsCSSRule needs to derives ScriptWrappableBase.

BUG=235436

Review URL: https://codereview.chromium.org/488063002

git-svn-id: svn://svn.chromium.org/blink/trunk@180645 bbb929c8-8fbe-4397-9dbb-9b2b20218538