[Discardable Shared Memory] Adding Windows support.

base/memory/discardable_shared_memory.h

 // Copyright 2014 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_DISCARDABLE_SHARED_MEMORY_H_
 #define BASE_MEMORY_DISCARDABLE_SHARED_MEMORY_H_
 
 #include "base/base_export.h"
 #include "base/logging.h"
 #include "base/memory/shared_memory.h"
 #include "base/threading/thread_collision_warner.h"
 #include "base/time/time.h"
 
 #if DCHECK_IS_ON()
 #include <set>
 #endif
 
+#if defined(OS_WIN)
+#include "base/win/windows_version.h"
+#endif
+
 // Linux (including Android) support the MADV_REMOVE argument with madvise()
 // which has the behavior of reliably causing zero-fill-on-demand pages to
 // be returned after a call. Here we define
 // DISCARDABLE_SHARED_MEMORY_ZERO_FILL_ON_DEMAND_PAGES_AFTER_PURGE on Linux
 // and Android to indicate that this type of behavior can be expected on
 // those platforms. Note that madvise() will still be used on other POSIX
 // platforms but doesn't provide the zero-fill-on-demand pages guarantee.
 #if defined(OS_LINUX) || defined(OS_ANDROID)
 #define DISCARDABLE_SHARED_MEMORY_ZERO_FILL_ON_DEMAND_PAGES_AFTER_PURGE
 #endif
@@ skipping 109 matching lines @@
   size_t mapped_size_;
   size_t locked_page_count_;
 #if DCHECK_IS_ON()
   std::set<size_t> locked_pages_;
 #endif
   // Implementation is not thread-safe but still usable if clients are
   // synchronized somehow. Use a collision warner to detect incorrect usage.
   DFAKE_MUTEX(thread_collision_warner_);
   Time last_known_usage_;
 
+#if defined(OS_WIN)
+  // Static Windows version.
+  // Not const as we'll first initialize, then lazy init this value.
+  static base::win::Version osVersion;
+#endif
+
   DISALLOW_COPY_AND_ASSIGN(DiscardableSharedMemory);
 };
 
 }  // namespace base
 
 #endif  // BASE_MEMORY_DISCARDABLE_SHARED_MEMORY_H_