Bug 1358893 - Stop using document.createEvent("beforeunloadevent") internally; r=smaug

We want to see if we can drop support due to web pages not using it.
Edge and Safari already don't support it, and Chrome reports that usage
is low enough that they're willing to drop it.  But telemetry doesn't
tell us about web usage if we're triggering it via our own internal
code.

MozReview-Commit-ID: 5YBfhQJExHC

--HG--
extra : rebase_source : fd359e3264ba96ef10617f4de767080c94b792fb

layout/base/nsDocumentViewer.cpp

@@ -18,7 +18,7 @@
 #include "nsIContentViewerContainer.h"
 #include "nsIContentViewer.h"
 #include "nsIDocumentViewerPrint.h"
-#include "nsIDOMBeforeUnloadEvent.h"
+#include "mozilla/dom/BeforeUnloadEvent.h"
 #include "nsIDocument.h"
 #include "nsPresContext.h"
 #include "nsIPresShell.h"
@@ -1185,12 +1185,13 @@ nsDocumentViewer::PermitUnloadInternal(bool *aShouldPrompt,
 
   // Now, fire an BeforeUnload event to the document and see if it's ok
   // to unload...
-  nsCOMPtr<nsIDOMDocument> domDoc = do_QueryInterface(mDocument);
-  nsCOMPtr<nsIDOMEvent> event;
-  domDoc->CreateEvent(NS_LITERAL_STRING("beforeunloadevent"),
-                      getter_AddRefs(event));
-  nsCOMPtr<nsIDOMBeforeUnloadEvent> beforeUnload = do_QueryInterface(event);
-  NS_ENSURE_STATE(beforeUnload);
+  nsIPresShell* shell = mDocument->GetShell();
+  nsPresContext* presContext = nullptr;
+  if (shell) {
+    presContext = shell->GetPresContext();
+  }
+  RefPtr<BeforeUnloadEvent> event =
+    new BeforeUnloadEvent(mDocument, presContext, nullptr);
   event->InitEvent(NS_LITERAL_STRING("beforeunload"), false, true);
 
   // Dispatching to |window|, but using |document| as the target.
@@ -1222,7 +1223,7 @@ nsDocumentViewer::PermitUnloadInternal(bool *aShouldPrompt,
 
   nsCOMPtr<nsIDocShell> docShell(mContainer);
   nsAutoString text;
-  beforeUnload->GetReturnValue(text);
+  event->GetReturnValue(text);
 
   // NB: we nullcheck mDocument because it might now be dead as a result of
   // the event being dispatched.

toolkit/components/telemetry/Histograms.json

@@ -377,9 +377,9 @@
   "CREATE_EVENT_BEFOREUNLOADEVENT" : {
     "alert_emails": ["ayg@aryeh.name"],
     "description": "Was document.createEvent(\"beforeunloadevent\") ever called",
-    "expires_in_version": "56",
+    "expires_in_version": "60",
     "kind": "count",
-    "bug_numbers": [1295588, 1251198]
+    "bug_numbers": [1295588, 1251198, 1358893]
   },
   "CREATE_EVENT_COMPOSITIONEVENT" : {
     "alert_emails": ["ayg@aryeh.name"],