Auto merge of #16845 - asajeffrey:script-rename-browsing-contexts, r=jdm

Renamed BrowsingContext to WindowProxy in script.

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Renamed `script::dom::BrowsingContext` to `script::dom::WindowProxy`.

The browsing context is mostly maintained in the constellation, not in script. It would be nice to rename `constellation::Frame` to `constellation::BrowsingContext`, but that will be very confusing if there are two `BrowsingContext` types.

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- [X] These changes do not require tests because renamings aren't externally visible

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