Bug 1855668 - Make ::backdrop inherit from the originating element. r=zrhoffman

This is as a result of a spec change (see link in the test), but it
addresses a very long-standing issue with this pseudo-element, see
https://github.com/whatwg/fullscreen/issues/124.

Differential Revision: https://phabricator.services.mozilla.com/D189484

layout/base/nsCSSFrameConstructor.cpp

@@ -1078,7 +1078,7 @@ void nsFrameConstructorState::ConstructBackdropFrameFor(nsIContent* aContent,
   RefPtr<ComputedStyle> style =
       mPresShell->StyleSet()->ResolvePseudoElementStyle(
           *aContent->AsElement(), PseudoStyleType::backdrop, nullptr,
-          /* aParentStyle */ nullptr);
+          aFrame->Style());
   MOZ_ASSERT(style->StyleDisplay()->mTopLayer == StyleTopLayer::Top);
   nsContainerFrame* parentFrame =
       GetGeometricParent(*style->StyleDisplay(), nullptr);