Factor out GamepadCommon base class from Gamepad and WebKitGamepad

Source/modules/gamepad/Gamepad.h

Index: Source/modules/gamepad/Gamepad.h
diff --git a/Source/modules/gamepad/Gamepad.h b/Source/modules/gamepad/Gamepad.h
index 1787b1f83d5f63f9f51d3b66a44da924078f5a65..a79fddd5c2536a7fdab2803de1cb5385e9cb9dfc 100644
--- a/Source/modules/gamepad/Gamepad.h
+++ b/Source/modules/gamepad/Gamepad.h
@@ -36,7 +36,7 @@
 
 namespace WebCore {
 
-class Gamepad FINAL : public RefCountedWillBeGarbageCollectedFinalized<Gamepad>, public ScriptWrappable {
+class Gamepad : public RefCountedWillBeGarbageCollectedFinalized<Gamepad>, public ScriptWrappable {
 public:
     static PassRefPtrWillBeRawPtr<Gamepad> create()
     {
@@ -69,7 +69,8 @@ public:
 
     void trace(Visitor*);
 
-private:
+protected:
+    // Inherited by WebKitGamepad
     Gamepad();
     String m_id;
     unsigned m_index;
@@ -77,7 +78,9 @@ private:
     unsigned long long m_timestamp;
     String m_mapping;
     FloatVector m_axes;
-    GamepadButtonVector m_buttons;
+
+private:
+    GamepadButtonVector m_buttons; // WebKitGamepad.buttons has different type
 };
 
 } // namespace WebCore