Factor out GamepadCommon base class from Gamepad and WebKitGamepad

Source/modules/gamepad/WebKitGamepadList.cpp

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "config.h"
 #include "modules/gamepad/WebKitGamepadList.h"
 
 namespace WebCore {
 
+WebKitGamepadList::WebKitGamepadList()
+{
+    ScriptWrappable::init(this);
+}
+
 WebKitGamepadList::~WebKitGamepadList()
 {
 }
 
 void WebKitGamepadList::set(unsigned index, PassRefPtrWillBeRawPtr<WebKitGamepad> gamepad)
 {
     if (index >= blink::WebGamepads::itemsLengthCap)
         return;
     m_items[index] = gamepad;
 }
 
-unsigned WebKitGamepadList::length() const
-{
-    return blink::WebGamepads::itemsLengthCap;
-}
-
 WebKitGamepad* WebKitGamepadList::item(unsigned index)
 {
     return index < length() ? m_items[index].get() : 0;
 }
 
 void WebKitGamepadList::trace(Visitor* visitor)
 {
     for (unsigned index = 0; index < blink::WebGamepads::itemsLengthCap; index++) {
         visitor->trace(m_items[index]);
     }
 }
 
 } // namespace WebCore