Support `UrlValuedAttribute` with CSS

[jeffkaufman]

Using UrlValuedAttribute with Stylesheet doesn't work:

~ $ curl http://www.jefftk.com/css-prefetch?PageSpeedFilters=
<link rel=stylesheet href="example.css">
<link rel=stylesheet css-prefetch="example.css">
<link css-prefetch="example.css">
<link rel=stylesheet css-prefetch="example.css" href="example.css">
<link css-prefetch="example.css" href="example.css">

~ $ curl http://www.jefftk.com/css-prefetch?PageSpeedFilters=rewrite_css
<link rel=stylesheet href="A.example.css.pagespeed.cf.ibS6ETNCIK.css">
<link rel=stylesheet css-prefetch="example.css">
<link css-prefetch="example.css">
<link rel=stylesheet css-prefetch="example.css" href="A.example.css.pagespeed.cf.ibS6ETNCIK.css">
<link css-prefetch="example.css" href="example.css">

~ $ curl http://www.jefftk.com/css-prefetch?PageSpeedFilters=extend_cache_css
<link rel=stylesheet href="example.css.pagespeed.ce.uEwpUZgfsK.css">
<link rel=stylesheet css-prefetch="example.css.pagespeed.ce.uEwpUZgfsK.css">
<link css-prefetch="example.css.pagespeed.ce.uEwpUZgfsK.css">
<link rel=stylesheet css-prefetch="example.css.pagespeed.ce.uEwpUZgfsK.css" href="example.css.pagespeed.ce.uEwpUZgfsK.css">
<link css-prefetch="example.css.pagespeed.ce.uEwpUZgfsK.css" href="example.css">

So adding that config does make us recognize it as css for extend_cache_css purposes but not for rewrite_css purposes.

It looks like for css we don't use resource_tag_scanner::ScanElement but instead implement a css-specific tag scanner in CssTagScanner::ParseCssElement.

It doesn't look like there's any config tweak that would make this work, or even a simple code tweak. Instead we would need to:

1. Modify CssTagScanner to call resource_tag_scanner to find out about any overrides it defines.

2. Modify all callers of CssTagScanner::ParseCssElement to accept getting multiple stylesheet urls in response.

[jeffkaufman]

This was reported on the mailing list: https://groups.google.com/forum/#!msg/mod-pagespeed-discuss/h6M9LIdsf2U/mEF2daOqBwAJ

[jeffkaufman]

Actually, I don't think CssTagScanner::ParseCssElement is the right place for this: that's for inlining, combining, and other fancy stuff. Someone telling us about a new kind of stylesheet attribute probably just wants us to optimize it in a stand-alone way, which means CssFilter::EndElementImpl is the place.