PSA: TabParents now also live in content processes

[Kan-Ru Chen (陳侃如)]

Hi all,

As a part of the ongoing nested content process work[1][2], the
TabParent now could be created in a content process so please don't
assume it's in the chrome process. Passing PBrowser around in ipdl
protocol is not safe either, because the PBrowser may live in a
different protocol tree. If you absolutely need to pass a PBrowser in
the protocol, please use PBrowserOrId (bug 1041419).

Before the testing infrastructure is ready[3] we could only track this
issue by hand. Many bugs has already been filed and tracked in [1]. The
feature is currently pref'ed off but reviewers please be aware of this
and suggest alternative way to implement if they use TabParent/PBrowser
incorrectly.

[1]: https://bugzilla.mozilla.org/show_bug.cgi?id=nested-oop
[2]: https://wiki.mozilla.org/Nested_Content_Processes
[3]: https://bugzilla.mozilla.org/show_bug.cgi?id=1038620

Kanru