Support radial-gradients with relative offsets

Source/core/css/CSSGradientValue.cpp

Index: Source/core/css/CSSGradientValue.cpp
diff --git a/Source/core/css/CSSGradientValue.cpp b/Source/core/css/CSSGradientValue.cpp
index 277162fd89ba49b8b354817c3e4bca249a68a349..632f97cdbf8d952aee0e3eb55bafc0f438994e38 100644
--- a/Source/core/css/CSSGradientValue.cpp
+++ b/Source/core/css/CSSGradientValue.cpp
@@ -29,6 +29,7 @@
 #include "core/CSSValueKeywords.h"
 #include "core/css/CSSCalculationValue.h"
 #include "core/css/CSSToLengthConversionData.h"
+#include "core/css/Pair.h"
 #include "core/dom/NodeRenderStyle.h"
 #include "core/dom/TextLinkColors.h"
 #include "core/rendering/RenderObject.h"
@@ -389,15 +390,31 @@ void CSSGradientValue::addStops(Gradient* gradient, const CSSToLengthConversionD
 
 static float positionFromValue(CSSPrimitiveValue* 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 (Pair* pair = value->getPairValue()) {
+        CSSValueID originID = 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;
+        }
+    }
+
     if (value->isNumber())
-        return value->getFloatValue() * conversionData.zoom();
+        return origin + sign * value->getFloatValue() * conversionData.zoom();
 
-    int edgeDistance = isHorizontal ? size.width() : size.height();
     if (value->isPercentage())
-        return value->getFloatValue() / 100.f * edgeDistance;
+        return origin + sign * value->getFloatValue() / 100.f * edgeDistance;
 
     if (value->isCalculatedPercentageWithLength())
-        return value->cssCalcValue()->toCalcValue(conversionData)->evaluate(edgeDistance);
+        return origin + sign * value->cssCalcValue()->toCalcValue(conversionData)->evaluate(edgeDistance);
 
     switch (value->getValueID()) {
     case CSSValueTop:
@@ -416,7 +433,7 @@ static float positionFromValue(CSSPrimitiveValue* value, const CSSToLengthConver
         break;
     }
 
-    return value->computeLength<float>(conversionData);
+    return origin + sign * value->computeLength<float>(conversionData);
 }
 
 FloatPoint CSSGradientValue::computeEndPoint(CSSPrimitiveValue* horizontal, CSSPrimitiveValue* vertical, const CSSToLengthConversionData& conversionData, const IntSize& size)