Slightly simplify building FillLayer objects

Source/core/css/resolver/AnimatedStyleBuilder.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 133 matching lines @@
 }
 
 template <CSSPropertyID property>
 void setOnFillLayers(FillLayer& fillLayers, const AnimatableValue* value, StyleResolverState& state)
 {
     const WillBeHeapVector<RefPtrWillBeMember<AnimatableValue> >& values = toAnimatableRepeatable(value)->values();
     ASSERT(!values.isEmpty());
     FillLayer* fillLayer = &fillLayers;
     FillLayer* prev = 0;
     for (size_t i = 0; i < values.size(); ++i) {
-        if (!fillLayer) {
-            switch (property) {
-            case CSSPropertyBackgroundImage:
-            case CSSPropertyBackgroundPositionX:
-            case CSSPropertyBackgroundPositionY:
-            case CSSPropertyBackgroundSize:
-            case CSSPropertyWebkitBackgroundSize:
-                fillLayer = new FillLayer(BackgroundFillLayer);
-                break;
-            case CSSPropertyWebkitMaskImage:
-            case CSSPropertyWebkitMaskPositionX:
-            case CSSPropertyWebkitMaskPositionY:
-            case CSSPropertyWebkitMaskSize:
-                fillLayer = new FillLayer(MaskFillLayer);
-                break;
-            default:
-                ASSERT_NOT_REACHED();
-            }
-            prev->setNext(fillLayer);
-        }
+        if (!fillLayer)
+            fillLayer = prev->ensureNext();
         const AnimatableValue* layerValue = values[i].get();
         switch (property) {
         case CSSPropertyBackgroundImage:
         case CSSPropertyWebkitMaskImage:
             if (layerValue->isImage()) {
                 fillLayer->setImage(state.styleImage(property, toAnimatableImage(layerValue)->toCSSValue()));
             } else {
                 ASSERT(toAnimatableUnknown(layerValue)->toCSSValueID() == CSSValueNone);
                 fillLayer->setImage(nullptr);
             }
@@ skipping 447 matching lines @@
         return;
     case CSSPropertyZoom:
         style->setZoom(clampTo<float>(toAnimatableDouble(value)->toDouble(), std::numeric_limits<float>::denorm_min()));
         return;
     default:
         ASSERT_NOT_REACHED();
     }
 }
 
 } // namespace blink