Auto merge of #15354 - cynicaldevil:manual-redirect, r=jdm

Redirect document loads manually

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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 #14596 .

r? @jdm

I ran some tests at random from the `navigating-across-documents` folder, and they are passing.

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components/script/dom/servoparser/mod.rs

@@ -527,6 +527,7 @@ impl Tokenizer {
 
 /// The context required for asynchronously fetching a document
 /// and parsing it progressively.
+#[derive(JSTraceable)]
 pub struct ParserContext {
     /// The parser that initiated the request.
     parser: Option<Trusted<ServoParser>>,