Instead of trying to locate the relevant StyleSheetList on style element
removal from the DOM, we now simply keep a pointer to the list instead.
This fixes an issue where using attachShadow() on an element that had
a declarative shadow DOM would cause any style elements present to use
the wrong StyleSheetList when removing themselves from the tree.
(cherry picked from commit 8543b8ad6a4d96bd0c247052572b8c2abf3929a7)
