Credential Manager: Add platform-exposed Credential objects.

Source/modules/credentialmanager/LocalCredential.h

 // 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.
 
 #ifndef LocalCredential_h
 #define LocalCredential_h
 
 #include "bindings/core/v8/ScriptWrappable.h"
 #include "bindings/core/v8/SerializedScriptValue.h"
 #include "modules/credentialmanager/Credential.h"
 #include "platform/heap/Handle.h"
 
 namespace blink {
 
 class LocalCredential FINAL : public Credential {
 public:
     static LocalCredential* create(const String& id, const String& name, const String& avatarURL, const String& password);
     virtual ~LocalCredential();
 
     // LocalCredential.idl
-    const String& password() const { return m_password; }
+    const String& password() const;
 
 private:
     LocalCredential(const String& id, const String& name, const String& avatarURL, const String& password);
-
-    String m_password;
 };
 
 } // namespace blink
 
 #endif // Credential_h