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

Index: third_party/WebKit/Source/core/html/HTMLFormElement.cpp
diff --git a/third_party/WebKit/Source/core/html/HTMLFormElement.cpp b/third_party/WebKit/Source/core/html/HTMLFormElement.cpp
index 53b47e973450e2b06b2e9d55df92b5d233e12752..491608592bab77800dda6f306d3f01daa1befdc2 100644
--- a/third_party/WebKit/Source/core/html/HTMLFormElement.cpp
+++ b/third_party/WebKit/Source/core/html/HTMLFormElement.cpp
@@ -350,9 +350,12 @@ void HTMLFormElement::submit(Event* event, bool activateSubmitButton)
     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;