Use ScopedTaskEnvironment instead of MessageLoopForUI in ui tests.

ui/app_list/pagination_model_unittest.cc

Index: ui/app_list/pagination_model_unittest.cc
diff --git a/ui/app_list/pagination_model_unittest.cc b/ui/app_list/pagination_model_unittest.cc
index fd5bcb2e1a57545fb7d993530a490c509ccd39b4..ec260e235e05a5eb53468388c1b8105c49b052cd 100644
--- a/ui/app_list/pagination_model_unittest.cc
+++ b/ui/app_list/pagination_model_unittest.cc
@@ -11,6 +11,7 @@
 #include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "base/strings/string_number_conversions.h"
+#include "base/test/scoped_task_environment.h"
 #include "testing/gtest/include/gtest/gtest.h"
 #include "ui/app_list/pagination_model_observer.h"
 
@@ -112,7 +113,9 @@ class TestPaginationModelObserver : public PaginationModelObserver {
 
 class PaginationModelTest : public testing::Test {
  public:
-  PaginationModelTest() {}
+  PaginationModelTest()
+      : scoped_task_environment_(
+            base::test::ScopedTaskEnvironment::MainThreadType::UI) {}
   ~PaginationModelTest() override {}
 
   // testing::Test overrides:
@@ -144,7 +147,7 @@ class PaginationModelTest : public testing::Test {
   TestPaginationModelObserver observer_;
 
  private:
-  base::MessageLoopForUI message_loop_;
+  base::test::ScopedTaskEnvironment scoped_task_environment_;
 
   DISALLOW_COPY_AND_ASSIGN(PaginationModelTest);
 };