Bug 1580766 - Add a unique ID for the BrowsingContext tree inside a browser element. r=kmag

This adds a `browserId` property to all browsing contexts. This ID is the same
for the entire tree of contexts inside a frame element. Each new top-level
context created for a given frame also inherits this ID. This allows identifying
the frame element for a given browsing context.

Originally authored by :mossop in D56245.

Differential Revision: https://phabricator.services.mozilla.com/D77911

dom/html/nsGenericHTMLFrameElement.cpp

@@ -208,6 +208,7 @@ void nsGenericHTMLFrameElement::UnbindFromTree(bool aNullParent) {
     mFrameLoader = nullptr;
   }
 
+  nsFrameLoaderOwner::UnbindFromTree();
   nsGenericHTMLElement::UnbindFromTree(aNullParent);
 }