Bug 1495363 - Abort the previous request, if a form is submitted twice, r=nike

uriloader/base/nsDocLoader.cpp

@@ -393,13 +393,12 @@ nsDocLoader::OnStartRequest(nsIRequest* request, nsISupports* aCtxt) {
   //
   if (mIsLoadingDocument) {
     if (loadFlags & nsIChannel::LOAD_DOCUMENT_URI) {
-      //
-      // Make sure that the document channel is null at this point...
-      // (unless its been redirected)
-      //
-      NS_ASSERTION(
-          (loadFlags & nsIChannel::LOAD_REPLACE) || !(mDocumentRequest.get()),
-          "Overwriting an existing document channel!");
+      // If we have a document request channel, and this is not a redirect, we
+      // must abort it and replace it with the new one.
+      if (!(loadFlags & nsIChannel::LOAD_REPLACE) && mDocumentRequest) {
+        mDocumentRequest->Cancel(NS_ERROR_ABORT);
+        mDocumentRequest = nullptr;
+      }
 
       // This request is associated with the entire document...
       mDocumentRequest = request;