Fix incorrect animation for some values of background-position.

Source/core/animation/css/CSSAnimatableValueFactory.cpp

 /*
  * 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
@@ skipping 157 matching lines @@
     case Contain:
     case Cover:
     case SizeNone:
         return AnimatableUnknown::create(CSSPrimitiveValue::create(fillSize.type));
     default:
         ASSERT_NOT_REACHED();
         return 0;
     }
 }
 
+inline static PassRefPtr<AnimatableValue> createFromBackgroundPosition(const Length& length, bool originIsSet, BackgroundEdgeOrigin origin, const RenderStyle& style)
+{
+    if (!originIsSet || origin == LeftEdge || origin == TopEdge)
+        return createFromLength(length, style);
+
+    return AnimatableLength::create(CSSCalcValue::createExpressionNode(
+        CSSCalcValue::createExpressionNode(CSSPrimitiveValue::create(100, CSSPrimitiveValue::CSS_PERCENTAGE), true),
+        CSSCalcValue::createExpressionNode(length, style.effectiveZoom()),
+        CalcSubtract));
+}
+
 template<CSSPropertyID property>
 inline static PassRefPtr<AnimatableValue> createFromFillLayers(const FillLayer* fillLayer, const RenderStyle& style)
 {
     ASSERT(fillLayer);
     Vector<RefPtr<AnimatableValue> > values;
     while (fillLayer) {
         if (property == CSSPropertyBackgroundImage || property == CSSPropertyWebkitMaskImage) {
             if (!fillLayer->isImageSet())
                 break;
             values.append(createFromStyleImage(fillLayer->image()));
         } else if (property == CSSPropertyBackgroundPositionX || property == CSSPropertyWebkitMaskPositionX) {
             if (!fillLayer->isXPositionSet())
                 break;
-            values.append(createFromLength(fillLayer->xPosition(), style));
+            values.append(createFromBackgroundPosition(fillLayer->xPosition(), fillLayer->isBackgroundXOriginSet(), fillLayer->backgroundXOrigin(), style));
         } else if (property == CSSPropertyBackgroundPositionY || property == CSSPropertyWebkitMaskPositionY) {
             if (!fillLayer->isYPositionSet())
                 break;
-            values.append(createFromLength(fillLayer->yPosition(), style));
+            values.append(createFromBackgroundPosition(fillLayer->yPosition(), fillLayer->isBackgroundYOriginSet(), fillLayer->backgroundYOrigin(), style));
         } else if (property == CSSPropertyBackgroundSize || property == CSSPropertyWebkitMaskSize) {
             if (!fillLayer->isSizeSet())
                 break;
             values.append(createFromFillSize(fillLayer->size(), style));
         } else {
             ASSERT_NOT_REACHED();
         }
         fillLayer = fillLayer->next();
     }
     return AnimatableRepeatable::create(values);
@@ skipping 243 matching lines @@
     case CSSPropertyZoom:
         return createFromDouble(style.zoom());
     default:
         ASSERT_NOT_REACHED();
         // This return value is to avoid a release crash if possible.
         return AnimatableUnknown::create(0);
     }
 }
 
 } // namespace WebCore