[webcrypto] Make WebCryptoKey nullable.

Source/core/platform/chromium/support/WebCrypto.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 48 matching lines @@
 }
 
 void WebCryptoResult::completeWithBoolean(bool b)
 {
     m_impl->completeWithBoolean(b);
     reset();
 }
 
 void WebCryptoResult::completeWithKey(const WebCryptoKey& key)
 {
+    ASSERT(!key.isNull());
     m_impl->completeWithKey(key);
     reset();
 }
 
 void WebCryptoResult::completeWithKeyPair(const WebCryptoKey& publicKey, const WebCryptoKey& privateKey)
 {
+    ASSERT(!publicKey.isNull());
+    ASSERT(!privateKey.isNull());
     m_impl->completeWithKeyPair(publicKey, privateKey);
     reset();
 }
 
 WebCryptoResult::WebCryptoResult(const WTF::PassRefPtr<WebCore::CryptoResult>& impl)
     : m_impl(impl)
 {
     ASSERT(m_impl.get());
 }
 
 void WebCryptoResult::reset()
 {
     m_impl.reset();
 }
 
 void WebCryptoResult::assign(const WebCryptoResult& o)
 {
     m_impl = o.m_impl;
 }
 
 } // namespace WebKit