Bug 1719285 - Make nsFrameLoader::GetExistingDocShell returning nsDocShell; r=smaug

Differential Revision: https://phabricator.services.mozilla.com/D119183

dom/base/nsFrameLoader.cpp

@@ -1561,10 +1561,8 @@ nsresult nsFrameLoader::SwapWithOtherLoader(nsFrameLoader* aOther,
     return NS_ERROR_DOM_SECURITY_ERR;
   }
 
-  RefPtr<nsDocShell> ourDocshell =
-      static_cast<nsDocShell*>(GetExistingDocShell());
-  RefPtr<nsDocShell> otherDocshell =
-      static_cast<nsDocShell*>(aOther->GetExistingDocShell());
+  RefPtr<nsDocShell> ourDocshell = GetExistingDocShell();
+  RefPtr<nsDocShell> otherDocshell = aOther->GetExistingDocShell();
   if (!ourDocshell || !otherDocshell) {
     // How odd
     return NS_ERROR_NOT_IMPLEMENTED;
@@ -3371,7 +3369,7 @@ already_AddRefed<Promise> nsFrameLoader::PrintPreview(
     sourceWindow =
         nsGlobalWindowOuter::Cast(aSourceBrowsingContext->GetDOMWindow());
   } else {
-    auto* ourDocshell = static_cast<nsDocShell*>(GetExistingDocShell());
+    nsDocShell* ourDocshell = GetExistingDocShell();
     if (NS_WARN_IF(!ourDocshell)) {
       promise->MaybeRejectWithNotSupportedError("No print preview docShell");
       return promise.forget();

dom/base/nsFrameLoader.h

@@ -137,7 +137,7 @@ class nsFrameLoader final : public nsStubMutationObserver,
   void StartDestroy(bool aForProcessSwitch);
   void DestroyDocShell();
   void DestroyComplete();
-  nsIDocShell* GetExistingDocShell() const { return mDocShell; }
+  nsDocShell* GetExistingDocShell() const { return mDocShell; }
   mozilla::dom::InProcessBrowserChildMessageManager*
   GetBrowserChildMessageManager() const {
     return mChildMessageManager;

dom/html/nsGenericHTMLFrameElement.cpp

@@ -244,8 +244,8 @@ nsresult nsGenericHTMLFrameElement::AfterSetAttr(
     if (aName == nsGkAtoms::scrolling) {
       if (mFrameLoader) {
         ScrollbarPreference pref = MapScrollingAttribute(aValue);
-        if (nsIDocShell* docshell = mFrameLoader->GetExistingDocShell()) {
-          nsDocShell::Cast(docshell)->SetScrollbarPreference(pref);
+        if (nsDocShell* docshell = mFrameLoader->GetExistingDocShell()) {
+          docshell->SetScrollbarPreference(pref);
         } else if (auto* child = mFrameLoader->GetBrowserBridgeChild()) {
           // NOTE(emilio): We intentionally don't deal with the
           // GetBrowserParent() case, and only deal with the fission iframe