Auto merge of #14089 - Wafflespeanut:tocss, r=SimonSapin

Make use of Servo-specific ToCss everywhere!

<!-- Please describe your changes on the following line: -->

This will allow types to be generic over our local `ToCss`

---
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- [ ] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors

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- [x] These changes do not require tests because it's a refactor

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ports/cef/Cargo.lock

@@ -2294,6 +2294,7 @@ dependencies = [
  "rustc-serialize 0.3.19 (registry+https://github.com/rust-lang/crates.io-index)",
  "serde 0.8.17 (registry+https://github.com/rust-lang/crates.io-index)",
  "serde_derive 0.8.17 (registry+https://github.com/rust-lang/crates.io-index)",
+ "url 1.2.0 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
 [[package]]

ports/geckolib/Cargo.lock

@@ -373,6 +373,7 @@ dependencies = [
  "cssparser 0.7.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "euclid 0.10.2 (registry+https://github.com/rust-lang/crates.io-index)",
  "rustc-serialize 0.3.19 (registry+https://github.com/rust-lang/crates.io-index)",
+ "url 1.2.0 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
 [[package]]

ports/geckolib/glue.rs

@@ -3,7 +3,7 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 use app_units::Au;
-use cssparser::{Parser, ToCss};
+use cssparser::Parser;
 use env_logger;
 use euclid::Size2D;
 use parking_lot::RwLock;
@@ -46,6 +46,7 @@ use style::sequential;
 use style::string_cache::Atom;
 use style::stylesheets::{Origin, Stylesheet};
 use style::timer::Timer;
+use style_traits::ToCss;
 use url::Url;
 
 /*

ports/geckolib/lib.rs

@@ -12,6 +12,7 @@ extern crate euclid;
 extern crate libc;
 #[macro_use] extern crate log;
 extern crate parking_lot;
+extern crate style_traits;
 extern crate url;
 
 #[allow(non_snake_case)]