Bug 1444991 - Part 5: Make some XPCOM methods more strongly typed, r=bz

This patch goes through and changes a bunch of places in our tree which mention
this bug to use the new feature, making the methods more strongly typed.

There are probably more places in tree which could be changed, but I didn't try
to find them.

dom/html/HTMLFormElement.cpp

@@ -979,7 +979,7 @@ HTMLFormElement::NotifySubmitObservers(nsIURI* aActionURL,
       nsCOMPtr<nsIFormSubmitObserver> formSubmitObserver(
                       do_QueryInterface(inst));
       if (formSubmitObserver) {
-        rv = formSubmitObserver->Notify(static_cast<nsIContent*>(this),
+        rv = formSubmitObserver->Notify(this,
                                         window ? window->GetCurrentInnerWindow() : nullptr,
                                         aActionURL,
                                         aCancelSubmit);
@@ -1977,7 +1977,7 @@ HTMLFormElement::CheckValidFormSubmission()
         observer = do_QueryInterface(inst);
 
         if (observer) {
-          observer->NotifyInvalidSubmit(static_cast<nsIContent*>(this),
+          observer->NotifyInvalidSubmit(this,
                                         static_cast<nsIArray*>(invalidElements));
         }
       }