Bug 1525887 - Add set of ContentParent subscribed to BrowsingContextGroup. r=nika

Add the origin ContentParent to a CanonicalBrowsingContext's group
when a CanonicalBrowsingContext is created from IPC. With this it is
possible to keep track of all child processes associated with a
BrowsingContextGroup.

Differential Revision: https://phabricator.services.mozilla.com/D19004

docshell/build/nsDocShellModule.cpp

@@ -16,7 +16,6 @@ namespace mozilla {
 // The one time initialization for this module
 nsresult InitDocShellModule() {
   mozilla::dom::BrowsingContext::Init();
-  mozilla::dom::BrowsingContextGroup::Init();
   nsresult rv = nsSHistory::Startup();
   NS_ENSURE_SUCCESS(rv, rv);