Share more code between hsl and hwb parsing.

[emilio]

No description provided.

[GPHemsley]

I'm not sure this was the best decision.

hsl() and hwb() are defined independently of each other in the spec. They already have different support for legacy color syntax (i.e. commas) in css-colors-4, and their definitions could further diverge in the future (e.g. relative colors in css-colors-5).

In addition, in combining the code like this, you lose the correlation between the variable names and the spec, which makes the code harder to read.

I think you should revert these changes and take a different approach:

• The spec explicitly defines the hue processing as being identical between HSL and HWB (for now, at least), so that could still be a candidate for factoring out.
• And HWB doesn't support comma syntax, so that code can between removed from the corresponding function.