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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Cargo.lock

@@ -3232,8 +3232,8 @@ dependencies = [
  "html5ever 0.21.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "parking_lot 0.4.8 (registry+https://github.com/rust-lang/crates.io-index)",
  "rayon 0.8.2 (registry+https://github.com/rust-lang/crates.io-index)",
- "rustc-serialize 0.3.24 (registry+https://github.com/rust-lang/crates.io-index)",
  "selectors 0.19.0",
+ "serde_json 1.0.2 (registry+https://github.com/rust-lang/crates.io-index)",
  "servo_arc 0.0.1",
  "servo_atoms 0.0.1",
  "servo_config 0.0.1",