Auto merge of #11875 - izgzhen:file-manager-backend, r=Manishearth

Integration and improvements of File API backends

Basically three major changes:

1. More complete origin check in `FileManagerThreadMsg`
2. Add reference counting logic to file manage store and script API
3. Integrate the support of slicing

r? @Manishearth

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- [ ] These changes fix #__ (github issue number if applicable).

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components/servo/Cargo.lock

@@ -1478,6 +1478,7 @@ dependencies = [
  "lazy_static 0.2.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "log 0.3.6 (registry+https://github.com/rust-lang/crates.io-index)",
  "msg 0.0.1",
+ "num-traits 0.1.32 (registry+https://github.com/rust-lang/crates.io-index)",
  "serde 0.7.11 (registry+https://github.com/rust-lang/crates.io-index)",
  "serde_macros 0.7.11 (registry+https://github.com/rust-lang/crates.io-index)",
  "url 1.1.1 (registry+https://github.com/rust-lang/crates.io-index)",