Auto merge of #15438 - asajeffrey:url-serializable-origin, r=jdm

Add ImmutableOrigin to allow for serializing origins

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This PR adds a serializable type `ImmutableOrigin` and a non-serializable type `MutableOrigin`. The immutable type represents an origin with `null` domain, and the mutable type represents an origin with a mutable domain. This separation is needed for implementing setting `document.domain`.

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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 #14892.
- [X] These changes do not require tests because it's a refactoring which will enable other features.

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