Fixed the behaviour of form submit to Firefox: form does not submit once it has been removed.

third_party/WebKit/Source/core/html/HTMLFormElement.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009 Apple Inc. All rights reserved.
  *           (C) 2006 Alexey Proskuryakov (ap@nypop.com)
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
@@ skipping 332 matching lines @@
         }
     }
 }
 
 void HTMLFormElement::submit(Event* event, bool activateSubmitButton)
 {
     FrameView* view = document().view();
     LocalFrame* frame = document().frame();
     if (!view || !frame || !frame->page())
         return;
+
+    // Do not submit if the form is not attached to the document.

[tkent, 2016/08/25 00:52:54]

I don't think this comment and the CL description should refer to Firefox
particularly because the new behavior matches to the standard and Edge too.

I recommend to add a comment to the beginning of this function like:

// https://html.spec.whatwg.org/multipage/forms.html#form-submission-algorithm
// 2. If form document is not connected, has no associated browsing context, or
// its active sandboxing flag set has its sandboxed forms browsing context flag
// set, then abort these steps without doing anything.

[iclelland, 2016/08/25 13:17:30]

Agreed -- that's better. (I looked for that kind of text at w3.org, but
https://www.w3.org/TR/html5/forms.html#form-submission-algorithm doesn't include
it)

Safari also appears to implement this behaviour.

[lunalu1, 2016/08/25 15:05:35]

Done.

+    // This matches the behaviour in Firefox.
     // See crbug.com/586749.

[tkent, 2016/08/25 00:52:54]

Remove this line.  The bug won't be so helpful after this CL.

[lunalu1, 2016/08/25 15:05:35]

Done.

     if (!isConnected())
-        UseCounter::count(document(), UseCounter::FormSubmissionNotInDocumentTree);

[tkent, 2016/08/25 00:52:54]

Need to remove FormSubmissionNotInDocumentTree from core/frame/UseCounter.h.

+        return;
 
     if (m_isSubmitting)
         return;
 
     // Delay dispatching 'close' to dialog until done submitting.
     EventQueueScope scopeForDialogClose;
     AutoReset<bool> submitScope(&m_isSubmitting, true);
 
     HTMLFormControlElement* firstSuccessfulSubmitButton = nullptr;
     bool needButtonActivation = activateSubmitButton; // do we need to activate a submit button?
@@ skipping 429 matching lines @@
 {
     for (const auto& control : associatedElements()) {
         if (!control->isFormControlElement())
             continue;
         if (toHTMLFormControlElement(control)->canBeSuccessfulSubmitButton())
             toHTMLFormControlElement(control)->pseudoStateChanged(CSSSelector::PseudoDefault);
     }
 }
 
 } // namespace blink