Fixes #3006 , (relates to) #648

From some analysis, it seems as if this issue only occurs with a only percentages in a calc.

• calc(10%) crashes.
• calc(10% + 10%) crashes.
• calc(10% + 0px) doesn't crash.
• calc(10vw) doesn't crash.

The resolve function returns a Length in the general case, but returns a Percentage if only percentages are provided.

As a sanity check, I looked at the code before it was changed (commit before change was here), and it only handled the case of percentages and lengths, as I do in this commit.

The old code being:

Optional<Length> CalculatedStyleValue::resolve_length_percentage(Length::ResolutionContext const& resolution_context, Length const& percentage_basis) const
{
    auto result = m_calculation->resolve(resolution_context, percentage_basis);

    return result.value().visit(
        [&](Length const& length) -> Optional<Length> {
            return length;
        },
        [&](Percentage const& percentage) -> Optional<Length> {
            return percentage_basis.percentage_of(percentage);
        },
        [&](auto const&) -> Optional<Length> {
            return {};
        });
}

Not sure if this is perfect, very open to constructive criticism.