Factor out GamepadCommon base class from Gamepad and WebKitGamepad

Source/modules/gamepad/GamepadList.h

Index: Source/modules/gamepad/GamepadList.h
diff --git a/Source/modules/gamepad/GamepadList.h b/Source/modules/gamepad/GamepadList.h
index b19e6afc5c042d2e2ff471da76bf39ceb9734733..57415d93be75df0a6ed1abf99c7bce7941eeb69a 100644
--- a/Source/modules/gamepad/GamepadList.h
+++ b/Source/modules/gamepad/GamepadList.h
@@ -47,12 +47,15 @@ public:
 
     void trace(Visitor*);
 
-private:
+protected:
+    // inherited by WebKitGamepadList
     GamepadList()
     {
         ScriptWrappable::init(this);
     }
 
+private:
+    // WebKitGamepadList items are of type WebKitGamepad, not Gamepad
     RefPtrWillBeMember<Gamepad> m_items[blink::WebGamepads::itemsLengthCap];
 };