Re-land: base: Fix DiscardableSharedMemory::mapped_size().

base/memory/discardable_shared_memory.cc

 // 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.
 
 #include "base/memory/discardable_shared_memory.h"
 
 #if defined(OS_POSIX)
 #include <unistd.h>
 #endif
 
@@ skipping 335 matching lines @@
   SharedState result(subtle::NoBarrier_Load(
       &SharedStateFromSharedMemory(shared_memory_)->value.i));
 
   return result.GetLockState() == SharedState::LOCKED ||
          !result.GetTimestamp().is_null();
 }
 
 void DiscardableSharedMemory::Close() {
   shared_memory_.Unmap();
   shared_memory_.Close();
+  mapped_size_ = 0;
 }
 
 Time DiscardableSharedMemory::Now() const {
   return Time::Now();
 }
 
 }  // namespace base