Because the image diff builder added classes directly on the diff table
element, and because the table element is reused across diff renders,
it was possible for the class to leak into the next diff render, which
may not have been an image diff. Apply the same styles without adding
classes to the table.
Bug: Issue 9651
Change-Id: Ie66a0dbd0577cce3a78c570d7055681271c5a70d