Add ScopedTaskEnvironment in extensions::ModuleSystemTest.

extensions/renderer/api_test_base.h

Index: extensions/renderer/api_test_base.h
diff --git a/extensions/renderer/api_test_base.h b/extensions/renderer/api_test_base.h
index 6433716c3ea64076d80d0413357cdc936f3e10e9..d74b23c6fb4574232685ccf2213df9d40142c2f1 100644
--- a/extensions/renderer/api_test_base.h
+++ b/extensions/renderer/api_test_base.h
@@ -9,7 +9,6 @@
 #include <string>
 #include <utility>
 
-#include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "extensions/renderer/module_system_test.h"
 #include "extensions/renderer/v8_schema_registry.h"
@@ -116,7 +115,6 @@ class ApiTestBase : public ModuleSystemTest {
   }
 
  private:
-  base::MessageLoop message_loop_;
   std::unique_ptr<ApiTestEnvironment> test_env_;
 };