Support persistent system profiles.

base/metrics/persistent_memory_allocator.h

 // Copyright (c) 2015 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_METRICS_PERSISTENT_MEMORY_ALLOCATOR_H_
 #define BASE_METRICS_PERSISTENT_MEMORY_ALLOCATOR_H_
 
 #include <stdint.h>
 
 #include <atomic>
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   // operation, such as how much memory is currently used. This should be
   // called before such information is to be displayed or uploaded.
   void UpdateTrackingHistograms();
 
   // While the above works much like malloc & free, these next methods provide
   // an "object" interface similar to new and delete.
 
   // Reserve space in the memory segment of the desired |size| and |type_id|.
   // A return value of zero indicates the allocation failed, otherwise the
   // returned reference can be used by any process to get a real pointer via
-  // the GetAsObject() or GetAsArray calls.
+  // the GetAsObject() or GetAsArray calls. The actual allocated size may be
+  // larger and will always be a multiple of 8 bytes (64 bits).
   Reference Allocate(size_t size, uint32_t type_id);
 
   // Allocate and construct an object in persistent memory. The type must have
   // both (size_t) kExpectedInstanceSize and (uint32_t) kPersistentTypeId
   // static constexpr fields that are used to ensure compatibility between
   // software versions. An optional size parameter can be specified to force
   // the allocation to be bigger than the size of the object; this is useful
   // when the last field is actually variable length.
   template <typename T>
   T* New(size_t size) {
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   // share the same pointer then an allocation on one will amount to an
   // allocation for all.
   volatile std::atomic<Reference>* const reference_;
 
   // No DISALLOW_COPY_AND_ASSIGN as it's okay to copy/move these objects.
 };
 
 }  // namespace base
 
 #endif  // BASE_METRICS_PERSISTENT_MEMORY_ALLOCATOR_H_