Bug #1: AbstractRange::(Mark|Unmark)Descendants should always use the shadow tree of web-exposed shadow root, instead of using light DOM elements of the host. Bug #2: aRange could possibly create mCrossShadowBoundaryRange first (due to boundaries are in different tree), and later moves the boundaries to the same tree. When this happens, we should remove mCrossShadowBoundaryRange and use the default range to represent it. Differential Revision: https://phabricator.services.mozilla.com/D207608