Linux sandbox: add basic Yama support

sandbox/linux/services/thread_helpers.h

Index: sandbox/linux/services/thread_helpers.h
diff --git a/sandbox/linux/services/thread_helpers.h b/sandbox/linux/services/thread_helpers.h
index 651e5d9c5013a21eae3d1c5b6ff8cd4ce56ad306..961f92e2b24788e63d2277310b92895458bd9063 100644
--- a/sandbox/linux/services/thread_helpers.h
+++ b/sandbox/linux/services/thread_helpers.h
@@ -14,8 +14,10 @@ namespace sandbox {
 class ThreadHelpers {
  public:
   // Check whether the current process is single threaded. |proc_self_tasks|
-  // should be a file descriptor to /proc/self/task/ and remains owned by the
-  // caller.
+  // can be a file descriptor to /proc/self/task/ and remains owned by the
+  // caller or -1.
+  // If |proc_self_tasks| is -1, this method will open /proc/self/task/ and
+  // crash if it cannot.
   static bool IsSingleThreaded(int proc_self_task);
 
   // Stop |thread| and ensure that it does not have an entry in