[Discardable Shared Memory] Adding Windows support.

base/memory/discardable_shared_memory.h

Index: base/memory/discardable_shared_memory.h
diff --git a/base/memory/discardable_shared_memory.h b/base/memory/discardable_shared_memory.h
index 0b5ac9fba8e5f7d9f82245720b69bd9c4051b49b..8be65c2a3c4657dd28ad2a7130f80747696e508b 100644
--- a/base/memory/discardable_shared_memory.h
+++ b/base/memory/discardable_shared_memory.h
@@ -144,6 +144,11 @@ class BASE_EXPORT DiscardableSharedMemory {
   DFAKE_MUTEX(thread_collision_warner_);
   Time last_known_usage_;
 
+  // Indicates whether Lock/Unlock pinning is supported on this platform.
+  // Call DoPinning() to access this information from within class functions.
+  static char pinning_supported_;
+  bool DoPinning();
+
   DISALLOW_COPY_AND_ASSIGN(DiscardableSharedMemory);
 };