[css-values-4] "Simplify a calculation tree" doesn't return in all paths

[AtkinsSJ]

Based on the current css-values-4 simplify a calculation tree algorithm.

Specifically, if root is an operator node that is not a calc-operator node, and there is not enough information to compute its operation, then we end up reaching the end of the algorithm.

For example, if we enter the algorithm with root being min(1em + 2em, 10px), and not enough information to convert an em to px:

• Steps 1 and 2 are skipped.
• Step 3, "At this point, root is an operator node. Simplify all the calculation children of root." will simplify the sum node. So we get min(3em, 10px).
• Step 4 does not have enough information to compute the min() operation, so it's skipped.
• No subsequent step handles a min() operation, so we go off the end of the algorithm.

I think the solution is just to add a catch-all final step of "Return root." I'll submit a PR momentarily.

Alternatively, step 4 could gain an "Otherwise, return root" substep. But this is complicated by the requirement to fall through to step 5 for Min and Max nodes. Personally I prefer the simpler solution.