Change first() and second() in CSSPairValue to return const references

Source/core/css/CSSGradientValue.cpp

 /*
  * Copyright (C) 2008 Apple Inc.  All rights reserved.
  * Copyright (C) 2015 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:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 471 matching lines @@
 
 static float positionFromValue(CSSValue* value, const CSSToLengthConversionData& conversionData, const IntSize& size, bool isHorizontal)
 {
     int origin = 0;
     int sign = 1;
     int edgeDistance = isHorizontal ? size.width() : size.height();
 
     // In this case the center of the gradient is given relative to an edge in the form of:
     // [ top | bottom | right | left ] [ <percentage> | <length> ].
     if (value->isValuePair()) {
-        CSSValuePair* pair = toCSSValuePair(value);
-        CSSValueID originID = toCSSPrimitiveValue(pair->first())->getValueID();
-        value = pair->second();
+        CSSValuePair& pair = toCSSValuePair(*value);
+        CSSValueID originID = toCSSPrimitiveValue(pair.first()).getValueID();
+        value = &pair.second();
 
         if (originID == CSSValueRight || originID == CSSValueBottom) {
             // For right/bottom, the offset is relative to the far edge.
             origin = edgeDistance;
             sign = -1;
         }
     }
 
     CSSPrimitiveValue* primitiveValue = toCSSPrimitiveValue(value);
 
@@ skipping 702 matching lines @@
     visitor->trace(m_firstRadius);
     visitor->trace(m_secondRadius);
     visitor->trace(m_shape);
     visitor->trace(m_sizingBehavior);
     visitor->trace(m_endHorizontalSize);
     visitor->trace(m_endVerticalSize);
     CSSGradientValue::traceAfterDispatch(visitor);
 }
 
 } // namespace blink