Every new PBrowser, whether it's created by the parent or the child, needs
to get a TabGroup assigned to it. That way IPC messages for the PBrowser will
be dispatched to that TabGroup.
For new PBrowsers created by the child, we just create a new TabGroup or reuse
the opener's TabGroup.
For PBrowsers created by the parent, the child needs to intercept the
PBrowserConstructor message and assign a TabGroup immediately. PBrowsers created
by the parent never have an opener so we can always create a new TabGroup.
In both cases, the nsGlobalWindow::TabGroupOuter logic needs to be updated to
read the TabGroup out of the IPC code. Otherwise the DOM and IPC code will get
out of sync about TabGroups.
MozReview-Commit-ID: D5iEdgirfvK
Bill McCloskey
Bevis Tseng