Reuse the check from IdentifierStyleValue in the CSS Parser, instead of
duplicating it. This might not be the ideal place to put it, but it
works for now.
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.
