Auto merge of #18710 - mhaessig:remove-rustc-serialize, r=jdm

Replace rustc_serialize with serde_json in style_tests

#12410 Stop using rustc_serialize

Replaced rustc_serialize with serde_json in
- [ ] ~~components/config/Cargo.toml~~
- [ ] ~~components/config/lib.rs~~
- [ ] ~~components/config/prefs.rs~~
- [ ] ~~components/script_traits/Cargo.toml~~
- [ ] ~~components/script_traits/lib.rs~~
- [ ] ~~components/script_traits/webdriver_msg.rs~~
- [ ] ~~components/webdriver_server/Cargo.toml~~
- [ ] ~~components/webdriver_server/lib.rs~~
- [X] tests/unit/style/Cargo.toml
- [X] test/unit/style/lib.rs
- [X] test/unit/style/properties/scaffolding.rs

PR checks:
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [X] These changes fix (partially) #12410.
- [X] These changes do not require tests because functionality was not changed or a test itself was edited

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tests/unit/style/lib.rs

@@ -11,8 +11,8 @@ extern crate euclid;
 #[macro_use] extern crate html5ever;
 extern crate parking_lot;
 extern crate rayon;
-extern crate rustc_serialize;
 extern crate selectors;
+extern crate serde_json;
 extern crate servo_arc;
 extern crate servo_atoms;
 extern crate servo_config;