Remove usage of ScopedAsyncTaskScheduler from BackgroundTaskRunnerTest.cpp.

ScopedAsyncTaskScheduler is deprecated.

Bug: 708584
Change-Id: Ia368d30ef1fd7b93519bf66845e5234505b0464c
Reviewed-on: https://chromium-review.googlesource.com/592164
Commit-Queue: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491269}

third_party/WebKit/Source/platform/threading/BackgroundTaskRunnerTest.cpp

@@ -5,7 +5,7 @@
 #include "platform/threading/BackgroundTaskRunner.h"
 
 #include <memory>
-#include "base/test/scoped_async_task_scheduler.h"
+#include "base/test/scoped_task_environment.h"
 #include "platform/CrossThreadFunctional.h"
 #include "platform/WaitableEvent.h"
 #include "platform/wtf/PtrUtil.h"
@@ -23,7 +23,7 @@ void PingPongTask(WaitableEvent* done_event) {
 }  // namespace
 
 TEST(BackgroundTaskRunnerTest, RunOnBackgroundThread) {
-  base::test::ScopedAsyncTaskScheduler scoped_async_task_scheduler;
+  base::test::ScopedTaskEnvironment scoped_task_environment;
   std::unique_ptr<WaitableEvent> done_event = WTF::MakeUnique<WaitableEvent>();
   BackgroundTaskRunner::PostOnBackgroundThread(
       BLINK_FROM_HERE,