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

extensions/common/stack_frame.h

 // Copyright 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 EXTENSIONS_COMMON_STACK_FRAME_H_
 #define EXTENSIONS_COMMON_STACK_FRAME_H_
 
 #include <stddef.h>
 
 #include <vector>
@@ skipping 11 matching lines @@
              const base::string16& source,
              const base::string16& function);
   ~StackFrame();
 
   // Construct a stack frame from a reported plain-text frame.
   static scoped_ptr<StackFrame> CreateFromText(
       const base::string16& frame_text);
 
   bool operator==(const StackFrame& rhs) const;
 
+  // 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 line numbers
+  // and column numbers shouldn't exceed UINT32_MAX even on 64 bit builds.
   uint32_t line_number;
   uint32_t column_number;
   base::string16 source;
   base::string16 function;  // optional
 };
 
 typedef std::vector<StackFrame> StackTrace;
 
 }  // namespace extensions
 
 #endif  // EXTENSIONS_COMMON_STACK_FRAME_H_