Use StaticAtomicSequenceNumber instead of subtle::Atomic32 in IPCMessage.

ipc/ipc_message.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 "ipc/ipc_message.h"
 
-#include "base/atomicops.h"
+#include "base/atomic_sequence_num.h"
 #include "base/logging.h"
 #include "build/build_config.h"
 
 #if defined(OS_POSIX)
 #include "ipc/file_descriptor_set_posix.h"
 #endif
 
 namespace {
 
-base::subtle::Atomic32 g_ref_num = 0;
+base::StaticAtomicSequenceNumber g_ref_num;
 
 // Create a reference number for identifying IPC messages in traces. The return
 // values has the reference number stored in the upper 24 bits, leaving the low
 // 8 bits set to 0 for use as flags.
 inline uint32 GetRefNumUpper24() {
   base::debug::TraceLog* trace_log = base::debug::TraceLog::GetInstance();
   int32 pid = trace_log ? trace_log->process_id() : 0;
-  int32 count = base::subtle::NoBarrier_AtomicIncrement(&g_ref_num, 1);
+  int32 count = g_ref_num.GetNext();

[tulloch, 2014/02/24 19:38:10]

Note that now `g_ref_num` counts from 0 instead of 1.

   // The 24 bit hash is composed of 14 bits of the count and 10 bits of the
   // Process ID. With the current trace event buffer cap, the 14-bit count did
   // not appear to wrap during a trace. Note that it is not a big deal if
   // collisions occur, as this is only used for debugging and trace analysis.
   return ((pid << 14) | (count & 0x3fff)) << 8;
 }
 
 }  // namespace
 
 namespace IPC {
@@ skipping 117 matching lines @@
 }
 
 void Message::EnsureFileDescriptorSet() {
   if (file_descriptor_set_.get() == NULL)
     file_descriptor_set_ = new FileDescriptorSet;
 }
 
 #endif
 
 }  // namespace IPC