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 25bb79e9741aa21524bbc83391caefc176057ea1..bfa87aedeb075ddf3a0b351c3ec5f4a66e182f96 100644
--- a/media/crypto/aes_decryptor.cc
+++ b/media/crypto/aes_decryptor.cc
@@ -302,6 +302,9 @@ 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_);