Issue #1118 - Part 5: Change the way document.open() works

This changes the work we do for document.open() in the following ways:
- We no longer create a new Window when doing document.open().
  We use the same Window but remove all the event listeners on the
  existing DOM tree and Window before removing the document's existing
  children to provide a clean slate document to use for .write().
- We no longer create a session history entry (previously would be a
  wyciwyg URI). We now replace the current one, effectively losing the
  entry for the original document.
- We now support document.open() on windowless documents.

layout/base/nsDocumentViewer.cpp

@@ -998,6 +998,9 @@ nsDocumentViewer::LoadComplete(nsresult aStatus)
   // will depend on whether it's cached!
   if(window &&
      (NS_SUCCEEDED(aStatus) || aStatus == NS_ERROR_PARSED_DATA_CACHED)) {
+    // If this code changes, the code in nsDocLoader::DocLoaderIsEmpty
+    // that fires load events for document.open() cases might need to
+    // be updated too.
     nsEventStatus status = nsEventStatus_eIgnore;
     WidgetEvent event(true, eLoad);
     event.mFlags.mBubbles = false;
@@ -1063,7 +1066,9 @@ nsDocumentViewer::LoadComplete(nsresult aStatus)
           MakeUnique<DocLoadingTimelineMarker>("document::Load"));
       }
 
+      d->SetLoadEventFiring(true);
       EventDispatcher::Dispatch(window, mPresContext, &event, nullptr, &status);
+      d->SetLoadEventFiring(false);
       if (timing) {
         timing->NotifyLoadEventEnd();
       }