[webcrypto] Make WebCryptoKey nullable.

Source/platform/exported/WebCryptoKey.cpp

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 45 matching lines @@
     const WebCryptoKeyUsageMask usages;
 };
 
 WebCryptoKey WebCryptoKey::create(WebCryptoKeyHandle* handle, WebCryptoKeyType type, bool extractable, const WebCryptoAlgorithm& algorithm, WebCryptoKeyUsageMask usages)
 {
     WebCryptoKey key;
     key.m_private = adoptRef(new WebCryptoKeyPrivate(adoptPtr(handle), type, extractable, algorithm, usages));
     return key;
 }
 
+WebCryptoKey WebCryptoKey::createNull()
+{
+    return WebCryptoKey();
+}
+
 WebCryptoKeyHandle* WebCryptoKey::handle() const
 {
+    ASSERT(!isNull());
     return m_private->handle.get();
 }
 
 WebCryptoKeyType WebCryptoKey::type() const
 {
+    ASSERT(!isNull());
     return m_private->type;
 }
 
 bool WebCryptoKey::extractable() const
 {
+    ASSERT(!isNull());
     return m_private->extractable;
 }
 
 const WebCryptoAlgorithm& WebCryptoKey::algorithm() const
 {
+    ASSERT(!isNull());
     return m_private->algorithm;
 }
 
 WebCryptoKeyUsageMask WebCryptoKey::usages() const
 {
+    ASSERT(!isNull());
     return m_private->usages;
 }
 
+bool WebCryptoKey::isNull() const
+{
+    return m_private.isNull();
+}
+
 void WebCryptoKey::assign(const WebCryptoKey& other)
 {
     m_private = other.m_private;
 }
 
 void WebCryptoKey::reset()
 {
     m_private.reset();
 }
 
 } // namespace WebKit