Allow creating a NaClAppThread without creating a new host OS thread

src/trusted/service_runtime/linux/thread_suspension.c

 /*
  * Copyright (c) 2012 The Native Client Authors. All rights reserved.
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 #include "native_client/src/trusted/service_runtime/linux/android_compat.h"
 
 #include <errno.h>
 #include <linux/futex.h>
 #include <signal.h>
@@ skipping 233 matching lines @@
      * Allocate register state struct if needed.  This is race-free
      * when we are called by NaClUntrustedThreadsSuspendAll(), since
      * that claims nap->threads_mu.
      */
     if (save_registers && natp->suspended_registers == NULL) {
       natp->suspended_registers = malloc(sizeof(*natp->suspended_registers));
       if (natp->suspended_registers == NULL) {
         NaClLog(LOG_FATAL, "NaClUntrustedThreadSuspend: malloc() failed\n");
       }
     }
-    if (pthread_kill(natp->thread.tid, NACL_THREAD_SUSPEND_SIGNAL) != 0) {
+    CHECK(natp->host_thread_is_defined);
+    if (pthread_kill(natp->host_thread.tid, NACL_THREAD_SUSPEND_SIGNAL) != 0) {
       NaClLog(LOG_FATAL, "NaClUntrustedThreadSuspend: "
               "pthread_kill() call failed\n");
     }
     WaitForUntrustedThreadToSuspend(natp);
   }
 }
 
 void NaClUntrustedThreadResume(struct NaClAppThread *natp) {
   Atomic32 old_state;
   Atomic32 new_state;
@@ skipping 35 matching lines @@
                                  NACL_APP_THREAD_SUSPENDING));
 
   if (natp->fault_signal == 0) {
     return 0;
   }
   *signal = natp->fault_signal;
   natp->fault_signal = 0;
   AtomicIncrement(&natp->nap->faulted_thread_count, -1);
   return 1;
 }