Instantiate ScopedTaskEnvironment in net unittests.

net/disk_cache/disk_cache_test_base.h

Index: net/disk_cache/disk_cache_test_base.h
diff --git a/net/disk_cache/disk_cache_test_base.h b/net/disk_cache/disk_cache_test_base.h
index b9564f930e24be20e367463d10d653de62a64e8f..aa91ed74c7f4d630d77eebf5f3d2fd8c1c7b6a01 100644
--- a/net/disk_cache/disk_cache_test_base.h
+++ b/net/disk_cache/disk_cache_test_base.h
@@ -12,7 +12,6 @@
 #include "base/files/file_path.h"
 #include "base/files/scoped_temp_dir.h"
 #include "base/macros.h"
-#include "base/test/scoped_async_task_scheduler.h"
 #include "base/threading/thread.h"
 #include "net/base/cache_type.h"
 #include "net/disk_cache/disk_cache.h"
@@ -56,13 +55,6 @@ class DiskCacheTest : public PlatformTest {
 
  private:
   base::ScopedTempDir temp_dir_;
-  std::unique_ptr<base::MessageLoop> message_loop_;
-
-  // Use a ScopedAsyncTaskScheduler instead of a ScopedTaskScheduler to allow
-  // disk_cache::InFlightIO::WaitForPendingIO to wait for TaskScheduler tasks
-  // from the main thread using WaitableEvents (this wouldn't work if
-  // TaskScheduler tasks ran on the main thread).
-  base::test::ScopedAsyncTaskScheduler scoped_async_task_scheduler_;
 };
 
 // Provides basic support for cache related tests.