Don't delay submit in onsubmit event handler

Source/core/html/HTMLFormElement.cpp

Index: Source/core/html/HTMLFormElement.cpp
diff --git a/Source/core/html/HTMLFormElement.cpp b/Source/core/html/HTMLFormElement.cpp
index 7e5a7859d87924d3238f3ac9ad3b22a6f35a131a..c24d0926c9bee5372d4314696cfa538fbe36a2fd 100644
--- a/Source/core/html/HTMLFormElement.cpp
+++ b/Source/core/html/HTMLFormElement.cpp
@@ -303,11 +303,12 @@ bool HTMLFormElement::prepareForSubmission(Event* event)
     RefPtr<FormState> formState = FormState::create(this, controlNamesAndValues, &document(), NotSubmittedByJavaScript);
     frame->loader().client()->dispatchWillSendSubmitEvent(formState.release());
 
+    // Set flag before submission as dispatchEvent could trigger another event
+    m_isSubmittingOrPreparingForSubmission = false;
+
     if (dispatchEvent(Event::createCancelableBubble(EventTypeNames::submit)))
         m_shouldSubmit = true;
 
-    m_isSubmittingOrPreparingForSubmission = false;
-
     if (m_shouldSubmit)
         submit(event, true, true, NotSubmittedByJavaScript);