Fix incorrect percentage top for positioned elements.

Source/core/css/ComputedStyleCSSValueMapping.cpp

Index: Source/core/css/ComputedStyleCSSValueMapping.cpp
diff --git a/Source/core/css/ComputedStyleCSSValueMapping.cpp b/Source/core/css/ComputedStyleCSSValueMapping.cpp
index 38f202ca7886e52498ddb9b05324a60635b9ef18..00dd034597ae67f7a1855dc7fa9d2efbf16236f6 100644
--- a/Source/core/css/ComputedStyleCSSValueMapping.cpp
+++ b/Source/core/css/ComputedStyleCSSValueMapping.cpp
@@ -170,10 +170,10 @@ static PassRefPtrWillBeRawPtr<CSSValue> valueForPositionOffset(const ComputedSty
         return nullptr;
     }
 
-    if (offset.isPercent() && renderer && renderer->isBox()) {
+    if (offset.isPercent() && renderer && renderer->isBox() && renderer->isPositioned()) {
         LayoutUnit containingBlockSize = (propertyID == CSSPropertyLeft || propertyID == CSSPropertyRight) ?
             toLayoutBox(renderer)->containingBlockLogicalWidthForContent() :
-            toLayoutBox(renderer)->containingBlockLogicalHeightForContent(ExcludeMarginBorderPadding);
+            toLayoutBox(renderer)->containingBlockLogicalHeightForGetComputedStyle();
         return zoomAdjustedPixelValue(valueForLength(offset, containingBlockSize), style);
     }
     if (offset.isAuto()) {