Clean up forward declarations in Source/platform

Source/platform/credentialmanager/PlatformCredential.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 PlatformCredential_h
 #define PlatformCredential_h
 
 #include "platform/heap/Handle.h"
 #include "platform/weborigin/KURL.h"
 #include "wtf/text/WTFString.h"
 
 namespace blink {
 
-class WebPlatformCredential;
-
 class PLATFORM_EXPORT PlatformCredential : public GarbageCollectedFinalized<PlatformCredential> {
     WTF_MAKE_NONCOPYABLE(PlatformCredential);
 public:
     static PlatformCredential* create(const String& id, const String& name, const KURL& avatarURL);
     virtual ~PlatformCredential();
 
     const String& id() const { return m_id; }
     const String& name() const { return m_name; }
     const KURL& avatarURL() const { return m_avatarURL; }
 
     virtual void trace(Visitor*) { }
 
 protected:
     PlatformCredential(const String& id, const String& name, const KURL& avatarURL);
 
 private:
     String m_id;
     String m_name;
     KURL m_avatarURL;
 };
 
 } // namespace blink
 
 #endif // PlatformCredential_h