Auto merge of #12924 - servo:selectors-ser, r=nox

Update selectors to 0.10, with ToCss serialization.

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r? @emilio

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- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

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components/layout/Cargo.toml

@@ -33,7 +33,7 @@ range = {path = "../range"}
 rustc-serialize = "0.3"
 script_layout_interface = {path = "../script_layout_interface"}
 script_traits = {path = "../script_traits"}
-selectors = {version = "0.9", features = ["heap_size"]}
+selectors = {version = "0.10", features = ["heap_size"]}
 serde_macros = "0.8"
 smallvec = "0.1"
 string_cache = {version = "0.2.23", features = ["heap_size"]}