Use ScopedTaskEnvironment instead of MessageLoopForUI in ViewsTestBase.

ui/views/widget/desktop_aura/desktop_native_widget_aura_unittest.cc

Index: ui/views/widget/desktop_aura/desktop_native_widget_aura_unittest.cc
diff --git a/ui/views/widget/desktop_aura/desktop_native_widget_aura_unittest.cc b/ui/views/widget/desktop_aura/desktop_native_widget_aura_unittest.cc
index 9c16dd93fb7191477e333a44a0e1eb451f300e98..b1027cf4eeeec0c8a53c5358af94d7998a6a2a38 100644
--- a/ui/views/widget/desktop_aura/desktop_native_widget_aura_unittest.cc
+++ b/ui/views/widget/desktop_aura/desktop_native_widget_aura_unittest.cc
@@ -8,6 +8,7 @@
 #include "base/macros.h"
 #include "base/run_loop.h"
 #include "base/single_thread_task_runner.h"
+#include "base/threading/thread_task_runner_handle.h"
 #include "build/build_config.h"
 #include "ui/aura/client/aura_constants.h"
 #include "ui/aura/client/cursor_client.h"
@@ -257,7 +258,7 @@ TEST_F(DesktopNativeWidgetAuraTest, WidgetCanBeDestroyedFromNestedLoop) {
   // |RunWithDispatcher()| below.
   base::RunLoop run_loop;
   base::Closure quit_runloop = run_loop.QuitClosure();
-  message_loop()->task_runner()->PostTask(
+  base::ThreadTaskRunnerHandle::Get()->PostTask(
       FROM_HERE,
       base::Bind(&QuitNestedLoopAndCloseWidget, base::Passed(&widget),
                  base::Unretained(&quit_runloop)));