Bug 1646567 - fix nsDocShell::HasUnloadedParent to work for OOP frames. r=nika

With fission enabled, walking the docshell tree to find unloaded
parents would stop at the first OOP parent. Instead we now walk
the `BrowsingContext`/`WindowContext` tree. To still check parents
which are OOP, we take advantage of the fact that a parent
`WindowContext` will no longer be the current `WindowContext` if
the parent was unloaded. For in process docshell's we additionally
check the docshell directly.

Differential Revision: https://phabricator.services.mozilla.com/D85328

docshell/base/nsDocShell.cpp

@@ -12509,14 +12509,20 @@ nsDocShell::GetAsyncPanZoomEnabled(bool* aOut) {
 }
 
 bool nsDocShell::HasUnloadedParent() {
-  RefPtr<nsDocShell> parent = GetInProcessParentDocshell();
-  while (parent) {
-    bool inUnload = false;
-    parent->GetIsInUnload(&inUnload);
-    if (inUnload) {
+  for (WindowContext* wc = GetBrowsingContext()->GetParentWindowContext(); wc;
+       wc->GetParentWindowContext()) {
+    if (wc->IsCached() || wc->IsDiscarded() ||
+        wc->GetBrowsingContext()->IsDiscarded()) {
+      // If a parent is OOP and the parent WindowContext is no
+      // longer current, we can assume the parent was unloaded.
+      return true;
+    }
+
+    if (wc->GetBrowsingContext()->IsInProcess() &&
+        (!wc->GetBrowsingContext()->GetDocShell() ||
+         wc->GetBrowsingContext()->GetDocShell()->GetIsInUnload())) {
       return true;
     }
-    parent = parent->GetInProcessParentDocshell();
   }
   return false;
 }

docshell/base/nsIDocShell.idl

@@ -460,7 +460,7 @@ interface nsIDocShell : nsIDocShellTreeItem
    * Find out whether the docshell is currently in the middle of a page
    * transition. This is set just before the pagehide/unload events fire.
    */
-  readonly attribute boolean isInUnload;
+  [infallible] readonly attribute boolean isInUnload;
 
   /**
    * Disconnects this docshell's editor from its window, and stores the