Default values for CSS vars fails

[eloypastor]

Default values for CSS vars fails when a default value es complex, like a gradient for example.

See this project

For this style

background: var(--fx-webkit-background, -webkit-gradient(linear, left top, left bottom, color-stop(0%,rgba(255,0,0,.65)), color-stop(100%,rgba(0,0,255,.65))));

the style result is

background: -webkit-gradient(linear, left top, left bottom, color-stop(0%,rgba(255,0,0,.65)), color-stop(100%,rgba(0,0,255,.65)))));

The last two parenthesis will be removed.

[zoechi]

probably a dup of #3221

[dfreedm]

Looks like this case is still a problem.

[dfreedm]

The variable matcher regex is at fault here, as it will match up until the comma between color stops. This regex is at peak complexity, so I think we will have to make a small custom state machine to replace it.

Visualization: http://regexr.com/3dl9j