Bug 1582832: Part 1 - Make FrameLoader owner rather than DocShell responsible for discarding a BC. r=nika

There are all sorts of lifecycle issues which arise from making DocShell
responsible for discarding BrowsingContexts. In this particular bug, we tend
to run into them in cases where we create a BrowsingContext for a FrameLoader,
and then never create a DocShell for it, leading to it never being destroyed.
But there are myriad other issues as well.

This patch moves the responsibility for BrowsingContext lifecycle management
to the FrameLoader/FrameLoaderOwner, rather than the DocShell, which makes
things more consistent, and more closely aligns with spec-defined behavior.

Differential Revision: https://phabricator.services.mozilla.com/D59008

--HG--
extra : moz-landing-system : lando