Add comments for struct members that were changed from size_t to uin32_t or uint64_t for IPC safety.

content/common/gpu/gpu_memory_uma_stats.h

 // Copyright (c) 2012 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 CONTENT_COMMON_GPU_GPU_MEMORY_UMA_STATS_H_
 #define CONTENT_COMMON_GPU_GPU_MEMORY_UMA_STATS_H_
 
 #include <stddef.h>
 
 namespace content {
 
 // Memory usage statistics send periodically to the browser process to report
 // in UMA histograms if the GPU process crashes.
+// Note: we use uint64_t instead of size_t for byte count because this struct
+// is sent over IPC which could span 32 & 64 bit processes.
 struct GPUMemoryUmaStats {
   GPUMemoryUmaStats()
       : bytes_allocated_current(0),
         bytes_allocated_max(0),
         context_group_count(0) {
   }
 
   // The number of bytes currently allocated.
-  uint32_t bytes_allocated_current;
+  uint64_t bytes_allocated_current;
 
   // The maximum number of bytes ever allocated at once.
-  uint32_t bytes_allocated_max;
+  uint64_t bytes_allocated_max;
 
   // The number of context groups.
   uint32_t context_group_count;
 };
 
 }  // namespace content
 
 #endif  // CONTENT_COMMON_GPU_GPU_MEMORY_UMA_STATS_H_