[tracing] Display the locked size of discardable memory segment.

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 142 matching lines @@
 }
 
 bool DiscardableSharedMemory::Unmap() {
   if (!shared_memory_.Unmap())
     return false;
 
   mapped_size_ = 0;
   return true;
 }
 
+size_t DiscardableSharedMemory::LockedSize() const {
+  if (!shared_memory_.memory())
+    return 0;
+
+  SharedState result(subtle::NoBarrier_Load(
+      &SharedStateFromSharedMemory(shared_memory_)->value.i));
+
+  return result.GetLockState() == SharedState::LOCKED
+             ? locked_page_count_ * base::GetPageSize()
+             : 0;
+}
+
 DiscardableSharedMemory::LockResult DiscardableSharedMemory::Lock(
     size_t offset, size_t length) {
   DCHECK_EQ(AlignToPageSize(offset), offset);
   DCHECK_EQ(AlignToPageSize(length), length);
 
   // Calls to this function must be synchronized properly.
   DFAKE_SCOPED_LOCK(thread_collision_warner_);
 
   DCHECK(shared_memory_.memory());
 
@@ skipping 189 matching lines @@
   NOTIMPLEMENTED();
 #endif
 }
 #endif
 
 Time DiscardableSharedMemory::Now() const {
   return Time::Now();
 }
 
 }  // namespace base