Use ScopedTaskEnvironment instead of MessageLoop in components unit tests.

components/history/core/browser/history_backend_unittest.cc

Index: components/history/core/browser/history_backend_unittest.cc
diff --git a/components/history/core/browser/history_backend_unittest.cc b/components/history/core/browser/history_backend_unittest.cc
index 4c03989081c5baeed60da5005933cd847be80ad7..4fba5854833f7add9030a516b85078e4729c83fe 100644
--- a/components/history/core/browser/history_backend_unittest.cc
+++ b/components/history/core/browser/history_backend_unittest.cc
@@ -30,6 +30,7 @@
 #include "base/strings/string_number_conversions.h"
 #include "base/strings/utf_string_conversions.h"
 #include "base/test/histogram_tester.h"
+#include "base/test/scoped_task_environment.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "build/build_config.h"
 #include "components/favicon_base/favicon_usage_data.h"
@@ -282,7 +283,7 @@ class HistoryBackendTestBase : public testing::Test {
   URLsModifiedList urls_modified_notifications_;
   URLsDeletedList urls_deleted_notifications_;
 
-  base::MessageLoop message_loop_;
+  base::test::ScopedTaskEnvironment scoped_task_environment_;
   base::FilePath test_dir_;
 
   DISALLOW_COPY_AND_ASSIGN(HistoryBackendTestBase);