Revert of file_manager: Migrate FILE thread to TaskScheduler

chrome/browser/chromeos/file_manager/file_browser_handlers.cc

Index: chrome/browser/chromeos/file_manager/file_browser_handlers.cc
diff --git a/chrome/browser/chromeos/file_manager/file_browser_handlers.cc b/chrome/browser/chromeos/file_manager/file_browser_handlers.cc
index 5f3df33371b42a59269bb5c14f6cd015233618f8..381dc8f28070926f2424be798060a2ea5023a600 100644
--- a/chrome/browser/chromeos/file_manager/file_browser_handlers.cc
+++ b/chrome/browser/chromeos/file_manager/file_browser_handlers.cc
@@ -16,7 +16,6 @@
 #include "base/macros.h"
 #include "base/memory/ptr_util.h"
 #include "base/strings/utf_string_conversions.h"
-#include "base/task_scheduler/post_task.h"
 #include "chrome/browser/chromeos/drive/file_system_util.h"
 #include "chrome/browser/chromeos/file_manager/app_id.h"
 #include "chrome/browser/chromeos/file_manager/fileapi_util.h"
@@ -219,6 +218,7 @@
     scoped_refptr<storage::FileSystemContext> file_system_context_handler,
     const scoped_refptr<const Extension>& handler_extension,
     const std::vector<FileSystemURL>& file_urls) {
+  DCHECK_CURRENTLY_ON(BrowserThread::FILE);
   DCHECK(handler_extension.get());
 
   storage::ExternalFileSystemBackend* backend =
@@ -290,9 +290,12 @@
   scoped_refptr<storage::FileSystemContext> file_system_context(
       util::GetFileSystemContextForExtensionId(profile_, extension_->id()));
 
-  base::PostTaskWithTraitsAndReplyWithResult(
-      FROM_HERE, {base::MayBlock(), base::TaskPriority::USER_BLOCKING},
-      base::Bind(&SetupFileAccessPermissions, file_system_context, extension_,
+  BrowserThread::PostTaskAndReplyWithResult(
+      BrowserThread::FILE,
+      FROM_HERE,
+      base::Bind(&SetupFileAccessPermissions,
+                 file_system_context,
+                 extension_,
                  file_urls),
       base::Bind(&FileBrowserHandlerExecutor::ExecuteAfterSetupFileAccess,
                  weak_ptr_factory_.GetWeakPtr()));