Fix two more bugs in ShadowDOM Selection
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
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1891783
gecko-commit: 0f54a84c32d1c22d71ff7307944b824639adbd6f
gecko-reviewers: jjaschke, smaug, dom-core