style: Use cbindgen for URIs.

This doesn't clean up as much as a whole, but it's a step in the right
direction. In particular, it allows us to start using simple bindings for:

 * Filters
 * Shapes and images, almost. Need to:
   * Get rid of the complex -moz- gradient parsing (let
     layout.css.simple-moz-gradient.enabled get to release).
 * Counters, almost. Need to:
   * Share the Attr representation with Gecko, by not using Option<>.
     * Just another variant should be enough (ContentItem::{Attr,Prefixedattr},
       maybe).

Which in turn allows us to remove a whole lot of bindings in followups to this.

The setup changes a bit. This also removes the double pointer I complained about
while reviewing the shared UA sheet patches. The old setup is:

```
SpecifiedUrl
 * CssUrl
   * Arc<CssUrlData>
     * String
     * UrlExtraData
 * UrlValueSource
   * Arc<CssUrlData>
   * load id
   * resolved uri
   * CORS mode.
   * ...
```

The new one removes the double reference to the url data via URLValue, and looks
like:

```
SpecifiedUrl
 * CssUrl
   * Arc<CssUrlData>
     * String
     * UrlExtraData
     * CorsMode
     * LoadData
       * load id
       * resolved URI
```

The LoadData is the only mutable bit that C++ can change, and is not used from
Rust. Ideally, in the future, we could just use rust-url to resolve the URL
after parsing or something, and make it all immutable. Maybe.

I've verified that this approach still works with the UA sheet patches (via the
LoadDataSource::Lazy).

The reordering of mWillChange is to avoid nsStyleDisplay from going over the
size limit. We want to split it up anyway in bug 1552587, but mBinding gains a
tag member, which means that we were having a bit of extra padding.

One thing I want to explore is to see if we can abuse rustc's non-zero
optimizations to predict the layout from C++, but that's something to explore at
some other point in time and with a lot of care and help from Michael (who sits
next to me and works on rustc ;)).

Differential Revision: https://phabricator.services.mozilla.com/D31742

components/style/values/generics/url.rs

@@ -5,6 +5,8 @@
 //! Generic types for url properties.
 
 /// An image url or none, used for example in list-style-image
+///
+/// cbindgen:derive-tagged-enum-copy-constructor=true
 #[derive(
     Animate,
     Clone,
@@ -21,16 +23,27 @@
     ToResolvedValue,
     ToShmem,
 )]
-pub enum UrlOrNone<Url> {
+#[repr(C, u8)]
+pub enum GenericUrlOrNone<U> {
     /// `none`
     None,
-    /// `A URL`
-    Url(Url),
+    /// A URL.
+    Url(U),
 }
 
+pub use self::GenericUrlOrNone as UrlOrNone;
+
 impl<Url> UrlOrNone<Url> {
     /// Initial "none" value for properties such as `list-style-image`
     pub fn none() -> Self {
         UrlOrNone::None
     }
+
+    /// Returns whether the value is `none`.
+    pub fn is_none(&self) -> bool {
+        match *self {
+            UrlOrNone::None => true,
+            UrlOrNone::Url(..) => false,
+        }
+    }
 }