Credential Manager: Add platform-exposed Credential objects.

Source/modules/credentialmanager/Credential.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/credentialmanager/Credential.h"
 
 namespace blink {
 
 Credential* Credential::create(const String& id, const String& name, const String& avatarURL)
 {
     return new Credential(id, name, avatarURL);
 }
 
+Credential::Credential(PlatformCredential* credential)
+    : m_private(credential)
+{
+}
+
 Credential::Credential(const String& id, const String& name, const String& avatarURL)
-    : m_id(id)
-    , m_name(name)
-    , m_avatarURL(avatarURL)
+    : m_private(PlatformCredential::create(id, name, avatarURL))
 {
     ScriptWrappable::init(this);
 }
 
+void Credential::trace(Visitor* visitor)
+{
+    visitor->trace(m_private);
+}
+
 Credential::~Credential()
 {
 }
 
 } // namespace blink