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

chrome/common/spellcheck_marker.h

 // Copyright (c) 2013 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 CHROME_COMMON_SPELLCHECK_MARKER_H_
 #define CHROME_COMMON_SPELLCHECK_MARKER_H_
 
 #include <stddef.h>
 #include <stdint.h>
 
@@ skipping 11 matching lines @@
     size_t text_length_;
   };
 
   // IPC requires a default constructor.
   SpellCheckMarker() : hash(0xFFFFFFFF), offset(UINT32_MAX) {}
 
   SpellCheckMarker(uint32_t hash, uint32_t offset)
       : hash(hash), offset(offset) {}
 
   uint32_t hash;
+  // Note: we use uint32_t instead of size_t because this struct is sent over
+  // IPC which could span 32 & 64 bit processes. This is fine since the offset
+  // shouldn't exceed UINT32_MAX even on 64 bit builds.
   uint32_t offset;
 };
 
 #endif  // CHROME_COMMON_SPELLCHECK_MARKER_H_