DocShells are associated with outer DOM Windows, rather than Documents, so
having the getter on the document is a bit odd to begin with. But it's also
considerably less convenient, since most of the times when we want a docShell
from JS, we're dealing most directly with a window, and have to detour through
the document to get it.
MozReview-Commit-ID: LUj1H9nG3QL
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extra : source : fcfb99baa0f0fb60a7c420a712c6ae7c72576871
extra : histedit_source : 5be9b7b29a52a4b8376ee0bdfc5c08b12e3c775a
031076f2f3