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 27272c67e03c1f15f1bbfbf2d7dbdbc4835655cf..bbeab8ccb1fcf7d6e6515dfbe195c06e265ce517 100644
--- a/media/crypto/aes_decryptor.cc
+++ b/media/crypto/aes_decryptor.cc
@@ -267,9 +267,8 @@ void AesDecryptor::Decrypt(const scoped_refptr<DecoderBuffer>& encrypted,
 
   DecryptionKey* key = GetKey(key_id);
   if (!key) {
-    // TODO(xhwang): Fire a need_key event here and add a test.
     DVLOG(1) << "Could not find a matching key for the given key ID.";
-    decrypt_cb.Run(kError, NULL);
+    decrypt_cb.Run(kNoKey, NULL);
     return;
   }
 
@@ -310,6 +309,9 @@ void AesDecryptor::Decrypt(const scoped_refptr<DecoderBuffer>& encrypted,
   decrypt_cb.Run(kSuccess, decrypted);
 }
 
+void AesDecryptor::Stop() {
+}
+
 void AesDecryptor::SetKey(const std::string& key_id,
                           scoped_ptr<DecryptionKey> decryption_key) {
   base::AutoLock auto_lock(key_map_lock_);