servo: Merge #18710 - Replace rustc_serialize with serde_json in style_tests (from mhaessig:remove-rustc-serialize); r=jdm

#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

Source-Repo: https://github.com/servo/servo
Source-Revision: 2977a4e26e0691dad8b640b47edbf77968d97d5a

--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : c4d6654736dd633c196658cedc3440097d72072c