Revert of file_manager: Migrate FILE thread to TaskScheduler

chrome/browser/chromeos/extensions/file_manager/file_manager_private_apitest.cc

Index: chrome/browser/chromeos/extensions/file_manager/file_manager_private_apitest.cc
diff --git a/chrome/browser/chromeos/extensions/file_manager/file_manager_private_apitest.cc b/chrome/browser/chromeos/extensions/file_manager/file_manager_private_apitest.cc
index c4413d1898a929ce82348c3f31cc2ba4187584ed..9db262ec3457641701d2ba47dcf9fe0a458b320e 100644
--- a/chrome/browser/chromeos/extensions/file_manager/file_manager_private_apitest.cc
+++ b/chrome/browser/chromeos/extensions/file_manager/file_manager_private_apitest.cc
@@ -15,6 +15,7 @@
 #include "chrome/browser/extensions/extension_apitest.h"
 #include "chrome/common/extensions/api/file_system_provider_capabilities/file_system_provider_capabilities_handler.h"
 #include "chrome/test/base/testing_profile.h"
+#include "chrome/test/base/ui_test_utils.h"
 #include "chromeos/dbus/cros_disks_client.h"
 #include "chromeos/disks/mock_disk_mount_manager.h"
 #include "components/drive/file_change.h"
@@ -378,11 +379,11 @@
       base::FilePath(FILE_PATH_LITERAL("/no-existing-fs-hash/root/aaa")),
       "extension_3", base::Bind(&AddFileWatchCallback));
 
-  // event_router->addFileWatch create some tasks which are performed on
-  // TaskScheduler. Wait until they are done.
-  base::TaskScheduler::GetInstance()->FlushForTesting();
-  // We also wait the UI thread here, since some tasks which are performed
-  // above message loop back results to the UI thread.
+  // event_router->addFileWatch create some tasks which are performed on message
+  // loop of BrowserThread::FILE. Wait until they are done.
+  content::RunAllPendingInMessageLoop(content::BrowserThread::FILE);
+  // We also wait the UI thread here, since some tasks which are performed above
+  // message loop back results to the UI thread.
   base::RunLoop().RunUntilIdle();
 
   // When /a is deleted (1 and 2 is notified).
@@ -420,9 +421,9 @@
       base::FilePath(FILE_PATH_LITERAL("/no-existing-fs/root/aaa")),
       "extension_3");
 
-  // event_router->addFileWatch create some tasks which are performed on
-  // TaskScheduler. Wait until they are done.
-  base::TaskScheduler::GetInstance()->FlushForTesting();
+  // event_router->removeFileWatch create some tasks which are performed on
+  // message loop of BrowserThread::FILE. Wait until they are done.
+  content::RunAllPendingInMessageLoop(content::BrowserThread::FILE);
 }
 
 IN_PROC_BROWSER_TEST_F(FileManagerPrivateApiTest, ContentChecksum) {