make base::SharedMemoryHandle a class on POSIX.

base/memory/shared_memory.h

 // 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.
 
 #ifndef BASE_MEMORY_SHARED_MEMORY_H_
 #define BASE_MEMORY_SHARED_MEMORY_H_
 
 #include <stddef.h>
 
 #include <string>
@@ skipping 288 matching lines @@
 #elif defined(OS_MACOSX) && !defined(OS_IOS)
   // The OS primitive that backs the shared memory region.
   SharedMemoryHandle shm_;
 
   // The mechanism by which the memory is mapped. Only valid if |memory_| is not
   // |nullptr|.
   SharedMemoryHandle::Type mapped_memory_mechanism_;
 
   int readonly_mapped_file_;
 #elif defined(OS_POSIX)
-  int                mapped_file_;
+  // The OS primitive that backs the shared memory region.
+  SharedMemoryHandle shm_;
   int                readonly_mapped_file_;
 #endif
   size_t             mapped_size_;
   void*              memory_;
   bool               read_only_;
   size_t             requested_size_;
 
   DISALLOW_COPY_AND_ASSIGN(SharedMemory);
 };
 
 }  // namespace base
 
 #endif  // BASE_MEMORY_SHARED_MEMORY_H_