SetFrame() is equivalent to `operator=`, so external callers can use `operator=`
instead. For the two callers wanting to set `nsFrameList` to `AbsoluteFrameList`
in `nsCSSFrameConstructor`, removing SetFrame() disallows it. However, we can
easily change the declaration from `nsFrameList` to a `AbsoluteFrameList` to
resolve the problem.
Differential Revision: https://phabricator.services.mozilla.com/D160840
a118547d80