Bug 1646563 - Remove BrowserId from nsFrameLoaderOwner, r=nika

We can just use BrowsingContext::BrowserId directly, so it's unnecessary to have
the field on nsFrameLoaderOwner as well.

This also makes it so that we only ever generate browser IDs in
BrowsingContext::CreatedDetached.

Differential Revision: https://phabricator.services.mozilla.com/D80121

dom/html/nsGenericHTMLFrameElement.cpp

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