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 730a9b80c70f03181f0e1eb21f52ceed5532447e..2ef711e245c789ec522df28a8b18f8376cc4fbbd 100644
--- a/media/crypto/aes_decryptor.cc
+++ b/media/crypto/aes_decryptor.cc
@@ -235,9 +235,8 @@ 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;
   }