Update from https://crrev.com/333737

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"
@@ skipping 30 matching lines @@
   virtual ~DiscardableSharedMemory();
 
   // Creates and maps a locked DiscardableSharedMemory object with |size|.
   // Returns true on success and false on failure.
   bool CreateAndMap(size_t size);
 
   // Maps the locked discardable memory into the caller's address space.
   // Returns true on success, false otherwise.
   bool Map(size_t size);
 
+  // Unmaps the discardable shared memory from the caller's address space.
+  // Returns true if successful; returns false on error or if the memory is
+  // not mapped.
+  bool Unmap();
+
   // The actual size of the mapped memory (may be larger than requested).
   size_t mapped_size() const { return mapped_size_; }
 
   // Returns a shared memory handle for this DiscardableSharedMemory object.
   SharedMemoryHandle handle() const { return shared_memory_.handle(); }
 
   // Locks a range of memory so that it will not be purged by the system.
   // The range of memory must be unlocked. The result of trying to lock an
   // already locked range is undefined. |offset| and |length| must both be
   // a multiple of the page size as returned by GetPageSize().
@@ skipping 75 matching lines @@
   // synchronized somehow. Use a collision warner to detect incorrect usage.
   DFAKE_MUTEX(thread_collision_warner_);
   Time last_known_usage_;
 
   DISALLOW_COPY_AND_ASSIGN(DiscardableSharedMemory);
 };
 
 }  // namespace base
 
 #endif  // BASE_MEMORY_DISCARDABLE_SHARED_MEMORY_H_