[webcrypto] Allow crypto operations to be cancelled by the platform implementation.

Source/platform/exported/WebCryptoResult.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 68 matching lines @@
 }
 
 void WebCryptoResult::completeWithKeyPair(const WebCryptoKey& publicKey, const WebCryptoKey& privateKey)
 {
     ASSERT(!publicKey.isNull());
     ASSERT(!privateKey.isNull());
     m_impl->completeWithKeyPair(publicKey, privateKey);
     reset();
 }
 
+bool WebCryptoResult::cancelled() const
+{
+    return m_impl->cancelled();
+}
+
 WebCryptoResult::WebCryptoResult(const 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 blink