Linux: add basic unprivileged namespace support.

sandbox/linux/tests/main.cc

Index: sandbox/linux/tests/main.cc
diff --git a/sandbox/linux/tests/main.cc b/sandbox/linux/tests/main.cc
index 81425456542c4cfeddea7afb74dc25202e5522b2..754b310dc4326cf8b8d4bed8c18c641f09f73ec1 100644
--- a/sandbox/linux/tests/main.cc
+++ b/sandbox/linux/tests/main.cc
@@ -2,9 +2,12 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+#include "base/at_exit.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 int main(int argc, char *argv[]) {
+  // The use of Callbacks requires an AtExitManager.
+  base::AtExitManager exit_manager;
   testing::InitGoogleTest(&argc, argv);
   // Always go through re-execution for death tests.
   // This makes gtest only marginally slower for us and has the