Bug 1555287 - Make sure to detach browsing context children early. r=nika

Waiting for docshells and frameloaders to destroy will leave attached
browsing contexts attached too long. In case the children of a
browsing contexts cannot be cached we want to detach all of them as
soon as possible.

Also normalizes the use of BrowsingContext::mGroup.

Differential Revision: https://phabricator.services.mozilla.com/D33602

docshell/base/nsDocShell.cpp

@@ -541,6 +541,8 @@ void nsDocShell::DestroyChildren() {
   }
 
   nsDocLoader::DestroyChildren();
+
+  mBrowsingContext->DetachChildren();
 }
 
 NS_IMPL_CYCLE_COLLECTION_INHERITED(nsDocShell, nsDocLoader,