Until a very recent "Fullscreen API" spec update [1], `::backdrop` pseudo-element was not supposed to inherit from any element, including its owner element.
But because some style-related functions expect that the parent style is never `nullptr` we use render view style as a parent style [2].
Since it's expected behavior for now across the codebase and the tests were not updated yet, we should use the same approach when creating `ResultionContext` for `::backdrop` pseudo-element.

[1] https://github.com/whatwg/fullscreen/commit/666438755d2f252be1d1f73d8214ac494489d32c
[2] https://github.com/WebKit/WebKit/blob/b872c8723cdbb67aafa46776c2e05066fd4ebbe2/Source/WebCore/rendering/updating/RenderTreeUpdaterGeneratedContent.cpp#L203C6-L204