Revert of TextRun::length() should return an unsigned

Source/platform/fonts/shaping/SimpleShaper.cpp

 /*
  * Copyright (C) 2003, 2006, 2008, 2009, 2010, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2008 Holger Hans Peter Freyther
  * Copyright (C) 2014 Google Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
@@ skipping 116 matching lines @@
                         if (m_forTextEmphasis)
                             glyphBuffer->add(fontData.zeroWidthSpaceGlyph(), &fontData, m_expansionPerOpportunity);
                         else
                             glyphBuffer->add(fontData.spaceGlyph(), &fontData, expansionAtThisOpportunity);
                     } else {
                         glyphBuffer->expandLastAdvance(expansionAtThisOpportunity);
                     }
                 }
             }
             if (m_run.allowsTrailingExpansion()
-                || (m_run.ltr() && charData.characterOffset + charData.clusterLength < m_run.length())
+                || (m_run.ltr() && charData.characterOffset + charData.clusterLength < static_cast<size_t>(m_run.length()))
                 || (m_run.rtl() && charData.characterOffset)) {
                 m_expansion -= m_expansionPerOpportunity;
                 width += m_expansionPerOpportunity;
                 m_isAfterExpansion = true;
             }
         } else {
             m_isAfterExpansion = false;
         }
 
         // Account for word spacing.
@@ skipping 106 matching lines @@
     float initialWidth = m_runWidthSoFar;
 
     if (!advance(m_currentCharacter + 1))
         return false;
 
     width = m_runWidthSoFar - initialWidth;
     return true;
 }
 
 } // namespace blink