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

media/crypto/aes_decryptor.cc

Index: media/crypto/aes_decryptor.cc
diff --git a/media/crypto/aes_decryptor.cc b/media/crypto/aes_decryptor.cc
index 552197bda9304bb9c3a999e98d6483c744cdbace..c0a7f881f3bbe5127174e89b287e918d0af37066 100644
--- a/media/crypto/aes_decryptor.cc
+++ b/media/crypto/aes_decryptor.cc
@@ -281,9 +281,8 @@ void AesDecryptor::Decrypt(const scoped_refptr<DecoderBuffer>& encrypted,
   }
 
   if (!key) {
-    // TODO(fgalligan): Fire a need_key event here and add a test.
     DVLOG(1) << "Could not find a matching key for given key ID.";
-    decrypt_cb.Run(kError, NULL);
+    decrypt_cb.Run(kNoKey, NULL);
     return;
   }