Implement "Key Presence" step in "Encrypted Block Encounted" algorithm in EME.

media/base/decryptor.h

 // Copyright (c) 2012 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 MEDIA_BASE_DECRYPTOR_H_
 #define MEDIA_BASE_DECRYPTOR_H_
 
 #include <string>
 
 #include "base/basictypes.h"
@@ skipping 16 matching lines @@
     kUnknownError = 1,
     kClientError,
     kServiceError,
     kOutputError,
     kHardwareChangeError,
     kDomainError
   };
 
   enum DecryptStatus {
     kSuccess,  // Decryption successfully completed. Decrypted buffer ready.
-    kError  // Error occurred during decryption.
+    kNoKey,  // No key is available to decrypt.
+    kError  // Key is available but an error occurred during decryption.
   };
 
   Decryptor() {}
   virtual ~Decryptor() {}
 
   // Generates a key request for the |key_system| with |init_data| provided.
   virtual void GenerateKeyRequest(const std::string& key_system,
                                   const uint8* init_data,
                                   int init_data_length) = 0;
 
@@ skipping 13 matching lines @@
                                 const std::string& session_id) = 0;
 
   // Decrypts the |encrypted| buffer. The decrypt status and decrypted buffer
   // are returned via the provided callback |decrypt_cb|. The |encrypted| buffer
   // must not be NULL.
   //
   // Note that the callback maybe called synchronously or asynchronously.
   //
   // If the returned status is kSuccess, the |encrypted| buffer is successfully
   // decrypted and the decrypted buffer is ready to be read.
+  // If the returned status is kNoKey, no decryption key is available to decrypt
+  // |encrypted| buffer. In this case the decrypted buffer must be NULL.
   // If the returned status is kError, unexpected error has occurred. In this
   // case the decrypted buffer must be NULL.
   typedef base::Callback<void(DecryptStatus,
                               const scoped_refptr<DecoderBuffer>&)> DecryptCB;
   virtual void Decrypt(const scoped_refptr<DecoderBuffer>& encrypted,
                        const DecryptCB& decrypt_cb) = 0;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(Decryptor);
 };
 
 }  // namespace media
 
 #endif  // MEDIA_BASE_DECRYPTOR_H_