Fix formAction return value for empty formaction content attribute.

Source/core/html/HTMLInputElement.idl

 /*
  * Copyright (C) 2006, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2006 Samuel Weinig <sam.weinig@gmail.com>
  * Copyright (C) 2012 Samsung Electronics. All rights reserved.
  *
  * 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
  * version 2 of the License, or (at your option) any later version.
  *
@@ skipping 18 matching lines @@
     [Reflect] attribute DOMString autocomplete;
     [Reflect] attribute boolean autofocus;
     [Reflect=checked] attribute boolean defaultChecked;
     attribute boolean checked;
     [Reflect] attribute DOMString dirName;
     [Reflect] attribute boolean disabled;
     [ImplementedAs=formOwner] readonly attribute HTMLFormElement? form;
     // The 'files' attribute is intentionally not readonly.
     // https://www.w3.org/Bugs/Public/show_bug.cgi?id=22682
     attribute FileList? files;
-    [Reflect, URL] attribute DOMString formAction;
+    attribute DOMString formAction;
     [CustomElementCallbacks] attribute DOMString formEnctype;
     [CustomElementCallbacks] attribute DOMString formMethod;
     [Reflect] attribute boolean formNoValidate;
     [Reflect] attribute DOMString formTarget;
     [CustomElementCallbacks] attribute unsigned long height;
     attribute boolean indeterminate;
     [RuntimeEnabled=InputModeAttribute, Reflect] attribute DOMString inputMode;
     readonly attribute HTMLElement? list;
     [Reflect] attribute DOMString max;
     [RaisesException=Setter, CustomElementCallbacks] attribute long maxLength;
@@ skipping 55 matching lines @@
     [Measure] attribute DOMString autocapitalize;
 
     // HTML Media Capture
     // http://www.w3.org/TR/html-media-capture/
     [RuntimeEnabled=MediaCapture, Reflect] attribute boolean capture;
 
     // Non-standard APIs
     [Reflect, MeasureAs=PrefixedDirectoryAttribute] attribute boolean webkitdirectory;
     [Reflect, MeasureAs=IncrementalAttribute] attribute boolean incremental;
 };