Implement [Unforgeable]
This is mostly stolen from Gecko. As there, we define the unforgeable members
on an object stored in the slots of the prototype object. They are then copied
onto instance objects when they are instantiated. It should be noted that
proxy objects see their unforgeable memebers defined on their expando object.
Unforgeable attributes aren't properly inherited in codegen (in a similar
fashion as getters and setters as filed in #5875) and require to be redefined
in derived interfaces. Fortunately, there are currently no such interfaces.
No unforgeable members can be included into the TestBinding interfaces for good
measure because they are not compatible with setters.
<!-- Reviewable:start -->
[<img src="https://reviewable.io/review_button.png" height=40 alt="Review on Reviewable"/>](https://reviewable.io/reviews/servo/servo/7988)
<!-- Reviewable:end -->
This is mostly stolen from Gecko. As there, we define the unforgeable members
on an object stored in the slots of the prototype object. They are then copied
onto instance objects when they are instantiated. It should be noted that
proxy objects see their unforgeable memebers defined on their expando object.
Unforgeable attributes aren't properly inherited in codegen (in a similar
fashion as getters and setters as filed in #5875) and require to be redefined
in derived interfaces. Fortunately, there are currently no such interfaces.
No unforgeable members can be included into the TestBinding interfaces for good
measure because they are not compatible with setters.
Given the unforgeable holder object has the same prototype as actual instances
of the interface, the finalize hook needs to check its slot pointer for nullity
before dropping it.
The new failing test isn't related to Unforgeable attributes, but to the fact
that all Document instances currently have a Location, even if their window
isn't in a browsing context.
Implement Range#deleteContents
Sadly calling ExtractContents and discarding the result doesn't do the right thing.
It may be worth having a CutContents method that takes an `Option<DocumentFragment>` and switch the behavior based on it, to share the code between DeleteContents and ExtractContents, like what Gecko does. Maybe a followup.
<!-- Reviewable:start -->
[<img src="https://reviewable.io/review_button.png" height=40 alt="Review on Reviewable"/>](https://reviewable.io/reviews/servo/servo/6826)
<!-- Reviewable:end -->
I'm working on resolving https://github.com/servo/servo/issues/8213 as per the spec online and feedback in the servo channel. Note that currently I cannot build (and thus test) my code, so this is a bit of a rough first draft. I'd still like feedback on my progress, and I hope that there is another way for my code to be tested.
<!-- Reviewable:start -->
[<img src="https://reviewable.io/review_button.png" height=40 alt="Review on Reviewable"/>](https://reviewable.io/reviews/servo/servo/8218)
<!-- Reviewable:end -->
