When serializing an sRGB color value that originated from a named color,
it should return the color name converted to ASCII lowercase. This
requires storing the color name (if it has one).
This change also requires explicitly removing the color names when
computing style, because computed color values do not retain their name.
It also requires removing a caching optimization in create_from_color(),
because adding the name means that the cached value might be wrong.
This fixes some WPT subtests, and also required updating some of our own
tests.
