Use ScopedTaskEnvironment instead of MessageLoopForUI in ui tests.

ui/base/dragdrop/os_exchange_data_unittest.cc

Index: ui/base/dragdrop/os_exchange_data_unittest.cc
diff --git a/ui/base/dragdrop/os_exchange_data_unittest.cc b/ui/base/dragdrop/os_exchange_data_unittest.cc
index b1425f161efefbb186ff30a7a0f9075b02838cb2..73454128eb9d28a17dec4a97d16666dc4f3cc501 100644
--- a/ui/base/dragdrop/os_exchange_data_unittest.cc
+++ b/ui/base/dragdrop/os_exchange_data_unittest.cc
@@ -5,9 +5,9 @@
 #include <memory>
 
 #include "base/files/file_util.h"
-#include "base/message_loop/message_loop.h"
 #include "base/pickle.h"
 #include "base/strings/utf_string_conversions.h"
+#include "base/test/scoped_task_environment.h"
 #include "build/build_config.h"
 #include "net/base/filename_util.h"
 #include "testing/gtest/include/gtest/gtest.h"
@@ -21,10 +21,12 @@ namespace ui {
 class OSExchangeDataTest : public PlatformTest {
  public:
   OSExchangeDataTest()
-      : event_source_(ui::PlatformEventSource::CreateDefault()) {}
+      : scoped_task_environment_(
+            base::test::ScopedTaskEnvironment::MainThreadType::UI),
+        event_source_(ui::PlatformEventSource::CreateDefault()) {}
 
  private:
-  base::MessageLoopForUI message_loop_;
+  base::test::ScopedTaskEnvironment scoped_task_environment_;
   std::unique_ptr<PlatformEventSource> event_source_;
 };