Fix incorrect percentage top for positioned elements.

Source/core/css/LayoutStyleCSSValueMapping.cpp

Index: Source/core/css/LayoutStyleCSSValueMapping.cpp
diff --git a/Source/core/css/LayoutStyleCSSValueMapping.cpp b/Source/core/css/LayoutStyleCSSValueMapping.cpp
index a33156932f6c3e183d3e8143a2626baca6d2b4a8..c9d004ecf898cb8bc007578ca04e442b0c5d13a3 100644
--- a/Source/core/css/LayoutStyleCSSValueMapping.cpp
+++ b/Source/core/css/LayoutStyleCSSValueMapping.cpp
@@ -162,10 +162,12 @@ static PassRefPtrWillBeRawPtr<CSSValue> valueForPositionOffset(const LayoutStyle
         return nullptr;
     }
 
-    if (offset.isPercent() && renderer && renderer->isBox()) {
+    if (offset.isPercent() && renderer && renderer->isBox() && renderer->isPositioned()) {
+        // http://www.w3.org/TR/CSS21/visudet.html#containing-block-details
+        AvailableLogicalHeightType heightType = (style.position() == AbsolutePosition) ? IncludePadding : ExcludeMarginBorderPadding;
         LayoutUnit containingBlockSize = (propertyID == CSSPropertyLeft || propertyID == CSSPropertyRight) ?
             toLayoutBox(renderer)->containingBlockLogicalWidthForContent() :
-            toLayoutBox(renderer)->containingBlockLogicalHeightForContent(ExcludeMarginBorderPadding);
+            toLayoutBox(renderer)->containingBlockLogicalHeightForContent(heightType);
         return zoomAdjustedPixelValue(valueForLength(offset, containingBlockSize), style);
     }
     if (offset.isAuto()) {