Use ScopedTaskEnvironment instead of MessageLoop in components unit tests.

components/captive_portal/captive_portal_detector_unittest.cc

Index: components/captive_portal/captive_portal_detector_unittest.cc
diff --git a/components/captive_portal/captive_portal_detector_unittest.cc b/components/captive_portal/captive_portal_detector_unittest.cc
index 28972bbd91085de4745ce41125fc007db8f323f4..7eaa099f2986969431b160ee3e35854b34bbf227 100644
--- a/components/captive_portal/captive_portal_detector_unittest.cc
+++ b/components/captive_portal/captive_portal_detector_unittest.cc
@@ -10,6 +10,7 @@
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/run_loop.h"
+#include "base/test/scoped_task_environment.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "base/time/time.h"
 #include "components/captive_portal/captive_portal_testing_utils.h"
@@ -113,7 +114,7 @@ class CaptivePortalDetectorTest : public testing::Test,
   }
 
  private:
-  base::MessageLoop message_loop_;
+  base::test::ScopedTaskEnvironment scoped_task_environment_;
   std::unique_ptr<CaptivePortalDetector> detector_;
 };