Merge 129370 - snapToSize rounds the incorrectly for negative locations

Source/WebCore/platform/FractionalLayoutUnit.h

 /*
  * Copyright (c) 2012, 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
@@ skipping 209 matching lines @@
     {
 #if ENABLE(SUBPIXEL_LAYOUT)
         if (m_value >= 0)
             return toInt();
         return (m_value - kFixedPointDenominator + 1) / kFixedPointDenominator;
 #else
         return m_value;
 #endif
     }
 
+    FractionalLayoutUnit fraction() const
+    {   
+        // Add the fraction to the size (as opposed to the full location) to avoid overflows.
+        // Compute fraction using the mod operator to preserve the sign of the value as it may affect rounding.
+        FractionalLayoutUnit fraction;
+        fraction.setRawValue(rawValue() % kFixedPointDenominator);
+        return fraction;
+    }
+
 #if ENABLE(SUBPIXEL_LAYOUT)
     static float epsilon() { return 1.0f / kFixedPointDenominator; }
 #else
     static int epsilon() { return 0; }
 #endif
     static const FractionalLayoutUnit max()
     {
         FractionalLayoutUnit m;
         m.m_value = std::numeric_limits<int>::max();
         return m;
@@ skipping 560 matching lines @@
 }
 
 inline float& operator/=(float& a, const FractionalLayoutUnit& b)
 {
     a = a / b;
     return a;
 }
 
 inline int snapSizeToPixel(FractionalLayoutUnit size, FractionalLayoutUnit location) 
 {
-    FractionalLayoutUnit fraction = location - location.floor();
+    FractionalLayoutUnit fraction = location.fraction();
     return (fraction + size).round() - fraction.round();
 }
 
 #if PLATFORM(QT)
 inline QDataStream& operator<<(QDataStream& stream, const FractionalLayoutUnit& value)
 {
     if (kFixedPointDenominator == 1)
         stream << value.rawValue();
     else
         stream << QString::fromLatin1("%1").arg(value.toFloat(), 0, 'f', 2);
 
     return stream;
 }
 
 inline QDataStream& operator>>(QDataStream& stream, FractionalLayoutUnit& value)
 {
     float v;
     stream >> v;
     value = v;
     return stream;
 }
 #endif
 
 } // namespace WebCore
 
 #endif // FractionalLayoutUnit_h