[cssom] Make CSSStyleRule serialization aware of nesting.

[tabatkins]

@emilio review please?

[emilio]

A bit weird that we're doing this fake indentation thing. So for nested declarations this would look like:

Source:

a {
  b {
    c {
      color: red;
    }
  }
}

cssText:

a {
  b {
  c { color: red; } }
}

Which is a bit off. I think I'd rather avoid newlines completely, but not particularly opposed.

[tabatkins]

I absolutely agree that this is busted, I'm just copying the existing indentation treatment for the other rules in the spec; @media already shows off that exact same awful behavior, for instance.

I didn't want to try and fix the indentation for just this rule and leave the rest busted, so I stuck with the current behavior and assume that all of them can be fixed up at once in a separate commit.

[emilio]

Sounds good to me, thanks.