Credential manager: Convert strings to URLs early.

Source/platform/exported/WebCredential.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 "public/platform/WebCredential.h"
 
 #include "platform/credentialmanager/PlatformCredential.h"
 
 namespace blink {
 
-WebCredential::WebCredential(const WebString& id, const WebString& name, const WebString& avatarURL)
+WebCredential::WebCredential(const WebString& id, const WebString& name, const WebURL& avatarURL)
     : m_platformCredential(PlatformCredential::create(id, name, avatarURL))
 {
 }
 
 void WebCredential::assign(const WebCredential& other)
 {
     m_platformCredential = other.m_platformCredential;
 }
 
 WebCredential::WebCredential(PlatformCredential* credential)
     : m_platformCredential(credential)
 {
 }
 
 void WebCredential::reset()
 {
     m_platformCredential.reset();
 }
 
 WebString WebCredential::id() const
 {
     return m_platformCredential->id();
 }
 
 WebString WebCredential::name() const
 {
     return m_platformCredential->name();
 }
 
-WebString WebCredential::avatarURL() const
+WebURL WebCredential::avatarURL() const
 {
     return m_platformCredential->avatarURL();
 }
 
 } // namespace blink