Use ScopedTaskEnvironment instead of MessageLoop in media unit tests.

media/cdm/aes_decryptor_unittest.cc

Index: media/cdm/aes_decryptor_unittest.cc
diff --git a/media/cdm/aes_decryptor_unittest.cc b/media/cdm/aes_decryptor_unittest.cc
index e5761903e9a56822d64b55ce6eda41f48e786604..5e80c79a8b43c8d6655dba6bc04e2618f7b9f542 100644
--- a/media/cdm/aes_decryptor_unittest.cc
+++ b/media/cdm/aes_decryptor_unittest.cc
@@ -14,6 +14,7 @@
 #include "base/json/json_reader.h"
 #include "base/macros.h"
 #include "base/run_loop.h"
+#include "base/test/scoped_task_environment.h"
 #include "base/values.h"
 #include "media/base/cdm_callback_promise.h"
 #include "media/base/cdm_config.h"
@@ -465,7 +466,7 @@ class AesDecryptorTest : public testing::TestWithParam<std::string> {
   // Helper class to load/unload External Clear Key Library, if necessary.
   std::unique_ptr<ExternalClearKeyTestHelper> helper_;
 
-  base::MessageLoop message_loop_;
+  base::test::ScopedTaskEnvironment scoped_task_environment_;
 
   // Constants for testing.
   const std::vector<uint8_t> original_data_;