Auto merge of #20265 - Xanewok:fix-js-objects-in-unions, r=jdm

Fix JS object conversion in unions

<!-- Please describe your changes on the following line: -->
Requires safe `Heap::boxed` constructor from https://github.com/servo/rust-mozjs/pull/395 (more info on it is in the PR).

Since unions currently assume that their respective members root themselves and can be stored on heap, I modified the union member object conversion branch to convert to a `RootedTraceableBox<Heap<*mut JSObject>>` (which is the currently generated type for objects in said unions).

I did it only for Unions and not dictionaries, since some dictionaries had bare `*mut JSObject` members - is this a mistake and something that needs further fixing?

Does this need a test, e.g. passing a union with object to a function that returns said object, and comparing the results for equality?

r? @jdm

Generated code with this patch (for `StringOrObject`):
```rust
impl FromJSValConvertible for StringOrObject {
    type Config = ();
    unsafe fn from_jsval(cx: *mut JSContext,
                         value: HandleValue,
                         _option: ())
                         -> Result<ConversionResult<StringOrObject>, ()> {
        if value.get().is_object() {
            match StringOrObject::TryConvertToObject(cx, value) {
                Err(_) => return Err(()),
                Ok(Some(value)) => return Ok(ConversionResult::Success(StringOrObject::Object(value))),
                Ok(None) => (),
            }

        }
        // (...)
    }
}

impl StringOrObject {
    // (...)
    unsafe fn TryConvertToObject(cx: *mut JSContext, value: HandleValue) -> Result<Option<RootedTraceableBox<Heap<*mut JSObject>>>, ()> {
        Ok(Some(RootedTraceableBox::from_box(Heap::boxed(value.get().to_object()))))
    }
}
```

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- [X] `./mach test-tidy` does not report any errors
- [ ] These changes fix #17011 (github issue number if applicable).

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Cargo.lock

@@ -1620,7 +1620,7 @@ dependencies = [
  "cssparser 0.23.2 (registry+https://github.com/rust-lang/crates.io-index)",
  "euclid 0.17.2 (registry+https://github.com/rust-lang/crates.io-index)",
  "hashglobe 0.1.0",
- "mozjs 0.1.11 (registry+https://github.com/rust-lang/crates.io-index)",
+ "mozjs 0.1.14 (registry+https://github.com/rust-lang/crates.io-index)",
  "selectors 0.19.0",
  "servo_arc 0.1.1",
  "smallbitvec 1.0.6 (registry+https://github.com/rust-lang/crates.io-index)",
@@ -1795,13 +1795,13 @@ dependencies = [
 
 [[package]]
 name = "mozjs"
-version = "0.1.11"
+version = "0.1.14"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 dependencies = [
  "cmake 0.1.29 (registry+https://github.com/rust-lang/crates.io-index)",
  "lazy_static 1.0.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "libc 0.2.39 (registry+https://github.com/rust-lang/crates.io-index)",
- "log 0.3.9 (registry+https://github.com/rust-lang/crates.io-index)",
+ "log 0.4.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "mozjs_sys 0.50.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "num-traits 0.1.37 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
@@ -2462,7 +2462,7 @@ dependencies = [
  "mime_guess 1.8.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "mitochondria 1.1.2 (registry+https://github.com/rust-lang/crates.io-index)",
  "mozangle 0.1.3 (registry+https://github.com/rust-lang/crates.io-index)",
- "mozjs 0.1.11 (registry+https://github.com/rust-lang/crates.io-index)",
+ "mozjs 0.1.14 (registry+https://github.com/rust-lang/crates.io-index)",
  "msg 0.0.1",
  "net_traits 0.0.1",
  "num-traits 0.1.37 (registry+https://github.com/rust-lang/crates.io-index)",
@@ -3844,7 +3844,7 @@ dependencies = [
 "checksum miow 0.2.1 (registry+https://github.com/rust-lang/crates.io-index)" = "8c1f2f3b1cf331de6896aabf6e9d55dca90356cc9960cca7eaaf408a355ae919"
 "checksum mitochondria 1.1.2 (registry+https://github.com/rust-lang/crates.io-index)" = "9de3eca27871df31c33b807f834b94ef7d000956f57aa25c5aed9c5f0aae8f6f"
 "checksum mozangle 0.1.3 (registry+https://github.com/rust-lang/crates.io-index)" = "4d916e4f2d39a00eeeb082ceb7c63c741e7c9d4f7915945f9225ae5e3b284092"
-"checksum mozjs 0.1.11 (registry+https://github.com/rust-lang/crates.io-index)" = "199f707066bf05b559ef6e46741c20e4f7bca8ae3a9c9d953d728dbb840f4eaa"
+"checksum mozjs 0.1.14 (registry+https://github.com/rust-lang/crates.io-index)" = "23e86a4da33f9325da4ccc652506284e00ff20f06d63f721eaebeb804eaca62d"
 "checksum mozjs_sys 0.50.1 (registry+https://github.com/rust-lang/crates.io-index)" = "e61a792a125b1364c5ec50255ed8343ce02dc56098f8868dd209d472c8de006a"
 "checksum mp3-metadata 0.3.0 (registry+https://github.com/rust-lang/crates.io-index)" = "4ab5f1d2693586420208d1200ce5a51cd44726f055b635176188137aff42c7de"
 "checksum mp4parse 0.9.1 (registry+https://github.com/rust-lang/crates.io-index)" = "f821e3799bc0fd16d9b861fb02fa7ee1b5fba29f45ad591dade105c48ca9a1a0"