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

ui/gfx/range/range.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 UI_GFX_RANGE_RANGE_H_
 #define UI_GFX_RANGE_RANGE_H_
 
 #include <stddef.h>
 #include <stdint.h>
 
@@ skipping 90 matching lines @@
   // is_reversed(), the range will get flipped when converted to an NSRange.
   NSRange ToNSRange() const;
 #elif defined(OS_WIN)
   CHARRANGE ToCHARRANGE() const;
 #endif
   // GTK+ has no concept of a range.
 
   std::string ToString() const;
 
  private:
+  // 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 text spans
+  // shouldn't exceed UINT32_MAX even on 64 bit builds.
   uint32_t start_;
   uint32_t end_;
 };
 
 GFX_EXPORT std::ostream& operator<<(std::ostream& os, const Range& range);
 
 }  // namespace gfx
 
 #endif  // UI_GFX_RANGE_RANGE_H_