Auto merge of #24576 - jaymodi98:iframe-srcdoc, r=jdm

Implement srcdoc support for iframes

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This PR contains changes related to adding srcdoc attribute parsing support for iframes in Servo. The following changes have been made:

- uncomment the [srcdoc](https://github.com/servo/servo/blob/f63b404e0cbf30380c4043700861110d06e548bb/components/script/dom/webidls/HTMLIFrameElement.webidl#L10-L11) WebIDL attribute, and implement the attribute getter.
- add a field to [LoadData](https://github.com/servo/servo/blob/e6b271d32981ad95442c029bf72bd35efc88f9c5/components/script_traits/lib.rs#L137-L164) for storing the srcdoc contents when loading a srcdoc iframe.
- (partially) implemented a new `page_load_about_srcdoc` method to `script_thread.rs` which loads the special `about:srcdoc` URL [per the specification](https://html.spec.whatwg.org/multipage/iframe-embed-object.html#process-the-iframe-attributes) and takes the srcdoc contents as an argument
- call this new method from [handle_new_layout](https://github.com/servo/servo/blob/e6b271d32981ad95442c029bf72bd35efc88f9c5/components/script/script_thread.rs#L2409-L2412) when it's detected that a srcdoc iframe is being loaded
- (partially) in [attribute_mutated](https://github.com/servo/servo/blob/e6b271d32981ad95442c029bf72bd35efc88f9c5/components/script/dom/htmliframeelement.rs#L560), ensure that changing the `srcdoc` attribute of an iframe element [follows the specification](https://html.spec.whatwg.org/multipage/iframe-embed-object.html#the-iframe-element:the-iframe-element-9).

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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 are a part of fix #4767  (GitHub issue number if applicable)

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