This will be needed to access the color of a stop from a SVG gradient
<stop> element (which does not participate in layout, so does not have
a layout node).
Now that LengthStyleValue never contains `auto`, IdentifierStyleValue is
the only type that can hold an identifier. This lets us remove a couple
of virtual methods from StyleValue.
I've kept `has_auto()` and `to_identifier()` for convenience, but they
are now simple non-virtual methods.
