Make SandboxIPCProcess a thread.

content/browser/zygote_host/zygote_host_impl_linux.cc

 // Copyright (c) 2012 The Chromium 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 "content/browser/zygote_host/zygote_host_impl_linux.h"
 
 #include <string.h>
 #include <sys/socket.h>
 #include <sys/stat.h>
 #include <sys/types.h>
@@ skipping 505 matching lines @@
   if (status != base::TERMINATION_STATUS_STILL_RUNNING) {
     ZygoteChildDied(handle);
   }
   return static_cast<base::TerminationStatus>(status);
 }
 
 pid_t ZygoteHostImpl::GetPid() const {
   return pid_;
 }
 
-pid_t ZygoteHostImpl::GetSandboxHelperPid() const {
-  return RenderSandboxHostLinux::GetInstance()->pid();
-}
-
 int ZygoteHostImpl::GetSandboxStatus() const {
   if (have_read_sandbox_status_word_)
     return sandbox_status_;
   return 0;
 }
 
 }  // namespace content