Use ScopedTaskEnvironment instead of MessageLoop in tests that use v8.

mojo/edk/js/tests/js_to_cpp_tests.cc

Index: mojo/edk/js/tests/js_to_cpp_tests.cc
diff --git a/mojo/edk/js/tests/js_to_cpp_tests.cc b/mojo/edk/js/tests/js_to_cpp_tests.cc
index d9fe22d22c1929f360f3f80a5cb34a73c9781be2..d193ffc9e8e73a5c57c12af822b1dc5d87bb70e5 100644
--- a/mojo/edk/js/tests/js_to_cpp_tests.cc
+++ b/mojo/edk/js/tests/js_to_cpp_tests.cc
@@ -12,9 +12,9 @@
 #include "base/files/file_path.h"
 #include "base/files/file_util.h"
 #include "base/macros.h"
-#include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "base/strings/utf_string_conversions.h"
+#include "base/test/scoped_task_environment.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "gin/array_buffer.h"
 #include "gin/public/isolate_holder.h"
@@ -418,7 +418,7 @@ class JsToCppTest : public testing::Test {
 
  private:
   base::ShadowingAtExitManager at_exit_;
-  base::MessageLoop loop;
+  base::test::ScopedTaskEnvironment scoped_task_environment_;
   base::RunLoop run_loop_;
 
   DISALLOW_COPY_AND_ASSIGN(JsToCppTest);