Recode shared memory usage on Windows, macOS and NaCl

base/memory/shared_memory_posix.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 "base/memory/shared_memory.h"
 
 #include <errno.h>
 #include <fcntl.h>
 #include <stddef.h>
 #include <sys/mman.h>
@@ skipping 280 matching lines @@
     memory_ = NULL;
   }
 
   return mmap_succeeded;
 }
 
 bool SharedMemory::Unmap() {
   if (memory_ == NULL)
     return false;
 
+  SharedMemoryTracker::GetInstance()->DecrementMemoryUsage(*this);
   munmap(memory_, mapped_size_);
-  SharedMemoryTracker::GetInstance()->DecrementMemoryUsage(*this);
   memory_ = NULL;
   mapped_size_ = 0;
   return true;
 }
 
 SharedMemoryHandle SharedMemory::handle() const {
   return shm_;
 }
 
 SharedMemoryHandle SharedMemory::TakeHandle() {
@@ skipping 40 matching lines @@
   return true;
 }
 #endif  // !defined(OS_ANDROID)
 
 SharedMemoryHandle SharedMemory::GetReadOnlyHandle() {
   CHECK(readonly_shm_.IsValid());
   return readonly_shm_.Duplicate();
 }
 
 }  // namespace base