TextRun::length() should return an unsigned

Source/platform/text/TextRunIterator.h

 // Copyright (C) 2013 Google Inc. All rights reserved.
 //
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //    * Redistributions of source code must retain the above copyright
 // notice, this list of conditions and the following disclaimer.
 //    * Redistributions in binary form must reproduce the above
 // copyright notice, this list of conditions and the following disclaimer
@@ skipping 51 matching lines @@
 
     bool operator==(const TextRunIterator& other)
     {
         return m_offset == other.m_offset && m_textRun == other.m_textRun;
     }
 
     bool operator!=(const TextRunIterator& other) { return !operator==(other); }
 
 private:
     const TextRun* m_textRun;
-    int m_offset;
+    unsigned m_offset;
 };
 
 
 } // namespace blink
 
 #endif // TextRunIterator_h