Linux Sandbox: check no threads before fork().

sandbox/linux/services/broker_process.cc

Index: sandbox/linux/services/broker_process.cc
diff --git a/sandbox/linux/services/broker_process.cc b/sandbox/linux/services/broker_process.cc
index 316883d260a5c43dc3ecb95909a7efd10cd8ad23..438e9726374707a59d6555b96e8009c195081cae 100644
--- a/sandbox/linux/services/broker_process.cc
+++ b/sandbox/linux/services/broker_process.cc
@@ -21,6 +21,7 @@
 #include "base/pickle.h"
 #include "base/posix/eintr_wrapper.h"
 #include "base/posix/unix_domain_socket_linux.h"
+#include "base/process/process_metrics.h"
 #include "build/build_config.h"
 #include "sandbox/linux/services/linux_syscalls.h"
 
@@ -146,6 +147,7 @@ bool BrokerProcess::Init(bool (*sandbox_callback)(void)) {
     return false;
   }
 
+  DCHECK_EQ(1, base::GetNumberOfThreads(base::GetCurrentProcessHandle()));
   int child_pid = fork();
   if (child_pid == -1) {
     close(socket_pair[0]);