Revert 240670 "Revert 239894 "Linux Sandbox: check no threads be..."

trunk/src/sandbox/linux/seccomp-bpf/sandbox_bpf.cc

Index: trunk/src/sandbox/linux/seccomp-bpf/sandbox_bpf.cc
===================================================================
--- trunk/src/sandbox/linux/seccomp-bpf/sandbox_bpf.cc	(revision 240957)
+++ trunk/src/sandbox/linux/seccomp-bpf/sandbox_bpf.cc	(working copy)
@@ -268,6 +268,10 @@
     SANDBOX_DIE("Process started without standard file descriptors");
   }
 
+  // This code is using fork() and should only ever run single-threaded.
+  // Most of the code below is "async-signal-safe" and only minor changes
+  // would be needed to support threads.
+  DCHECK(IsSingleThreaded(proc_fd_));
   pid_t pid = fork();
   if (pid < 0) {
     // Die if we cannot fork(). We would probably fail a little later