CSSMin: Correctly format 'url()' values with parentheses etc.

Introduce new static function, CSSMin::buildUrlValue.

Actually using such values in CSS does not work well because the
URL_REGEX is nowhere near good enough. :(

Change-Id: I04a7078dd0087bcb461fa5e5168c870d37c255f4
Merge "CSSMin: Correctly format 'url()' values with parentheses etc."