Enable one PID namespace per process for NaCl processes.

sandbox/linux/services/namespace_sandbox.cc

Index: sandbox/linux/services/namespace_sandbox.cc
diff --git a/sandbox/linux/services/namespace_sandbox.cc b/sandbox/linux/services/namespace_sandbox.cc
index 23796446f3044e88a6ddcb5d6e38fb41156e8456..a1e71f59b31f1b77185b64f60086d0bace80c149 100644
--- a/sandbox/linux/services/namespace_sandbox.cc
+++ b/sandbox/linux/services/namespace_sandbox.cc
@@ -77,7 +77,7 @@ void TerminationSignalHandler(int sig) {
     _exit(g_signal_exit_codes[sig_idx]);
   }
 
-  _exit(NamespaceSandbox::kDefaultExitCode);
+  _exit(SignalExitCode(sig));
 }
 #endif  // !defined(OS_NACL_NONSFI)
 
@@ -157,7 +157,7 @@ void NamespaceSandbox::InstallDefaultTerminationSignalHandlers() {
   };
 
   for (const int sig : kDefaultTermSignals) {
-    InstallTerminationSignalHandler(sig, kDefaultExitCode);
+    InstallTerminationSignalHandler(sig, SignalExitCode(sig));
   }
 }