Cleanup: Convert chrome_browser_main_linux.cc to use the blocking thread pool.

chrome/browser/sxs_linux.h

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CHROME_BROWSER_SXS_LINUX_H_
 #define CHROME_BROWSER_SXS_LINUX_H_
 
 #include "base/compiler_specific.h"
 
 namespace base {
 class FilePath;
 }
 
 namespace sxs_linux {
 
 // Records channel of the running browser in user data dir. This is needed
 // to support a seamless and automatic upgrade from non-side-by-side
 // to side-by-side Linux packages (the latter use different default data dirs).
-// Must be run on FILE thread.
+// Must be run on a blocking thread pool.
 void AddChannelMarkToUserDataDir();
 
 // Returns true if user data dir migration has been requested.
 bool ShouldMigrateUserDataDir() WARN_UNUSED_RESULT;
 
 // Migrates user data dir to a side-by-side-compatible one.
 // Returns exit code - caller should make the process exit with that code.
 int MigrateUserDataDir() WARN_UNUSED_RESULT;
 
 }  // namespace sxs_linux
 
 #endif  // CHROME_BROWSER_SXS_LINUX_H_