Fix style sheet parsing

This is for #9875 .

When minimizing style sheets for parsing, limit string removal to content property.

Added unit tests to CSSUtils and Quick View.

I'm a bit confused about where it's crashing and what's causing the crash... The *old* code removed the gradient entirely:

```javascript
> CSSU.reduceStyleSheetForRegExParsing('            .uploading-item{style: "background: linear-gradient(to right, #e8eef3, {{this.percent}}%, transparent 100%;"}')
"            .uploading-item{style:}"
```

The new code would actually leave it in. The new regular expression doesn't match anything in the file.