Update MediaKeySession lifetime description to be Oilpan reality-based.

Source/modules/crypto/CryptoResultImpl.h

 /*
  * 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 18 matching lines @@
  */
 
 #ifndef CryptoResultImpl_h
 #define CryptoResultImpl_h
 
 #include "bindings/core/v8/ScriptPromise.h"
 #include "core/dom/ExceptionCode.h"
 #include "platform/CryptoResult.h"
 #include "public/platform/WebCrypto.h"
 #include "wtf/Forward.h"
-#include "wtf/WeakPtr.h"
 
 namespace blink {
 
 class ScriptPromiseResolver;
 ExceptionCode webCryptoErrorToExceptionCode(WebCryptoErrorType);
 
 // Wrapper around a Promise to notify completion of the crypto operation.
 //
 // The thread on which CryptoResultImpl was created on is referred to as the
 // "origin thread".
@@ skipping 30 matching lines @@
 
     // FIXME: ScriptPromiseResolver should not be exported.
     // Instead, use ScriptPromise.
     ScriptPromiseResolver* m_resolver;
     volatile int m_cancelled;
 };
 
 } // namespace blink
 
 #endif