Auto merge of #20320 - kwonoj:feat-wr-revision, r=jdm

feat(capture_webrender): write webrender revision into text

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Relates to https://github.com/servo/servo/pull/20315#issuecomment-373819735.

This PR try to generate `wr.txt` when trigger webrender capture. By reading gecko's implementation at [here](3b8e63c66a/gfx/doc/README.webrender (L53)), it seems gecko's build script generates txt file for containing revision of webrender and read it each time trigger capturing.

In this PR tries to similar in cruxwise with small differences:
- `cargo build` reads `cargo.lock`, export it into `${OUT_DIR}/`, included via macro in build time.
- when capturing triggered, those revision will be written as `wr.txt`.

Probably point of discussion & need to be updated in PR if necessary:
~- Is it acceptable `mach` generates module file on build bootstrapping? Should there be other recommendation?~ Now cargo build takes care of generation.

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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #20295 (github issue number if applicable).

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- [ ] There are tests for these changes OR
- [ ] These changes do not require tests because _____

- This PR manually verified on local mac OS machine.

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components/compositing/Cargo.toml

@@ -4,6 +4,7 @@ version = "0.0.1"
 authors = ["The Servo Project Developers"]
 license = "MPL-2.0"
 publish = false
+build = "build.rs"
 
 [lib]
 name = "compositing"
@@ -29,3 +30,6 @@ style_traits = {path = "../style_traits"}
 time = "0.1.17"
 webrender = {git = "https://github.com/servo/webrender", features = ["capture"]}
 webrender_api = {git = "https://github.com/servo/webrender", features = ["ipc"]}
+
+[build-dependencies]
+toml = "0.4.5"