Expose custom property values to getComputedStyle.

third_party/WebKit/Source/core/css/ComputedStyleCSSValueMapping.cpp

Index: third_party/WebKit/Source/core/css/ComputedStyleCSSValueMapping.cpp
diff --git a/third_party/WebKit/Source/core/css/ComputedStyleCSSValueMapping.cpp b/third_party/WebKit/Source/core/css/ComputedStyleCSSValueMapping.cpp
index 27e94c1c960cfbbddb30e11ab4d2babdc55b6f5f..c639af296431dde43941b89f0b5be588d7e99668 100644
--- a/third_party/WebKit/Source/core/css/ComputedStyleCSSValueMapping.cpp
+++ b/third_party/WebKit/Source/core/css/ComputedStyleCSSValueMapping.cpp
@@ -32,6 +32,7 @@
 #include "core/css/CSSBorderImageSliceValue.h"
 #include "core/css/CSSCounterValue.h"
 #include "core/css/CSSCustomIdentValue.h"
+#include "core/css/CSSCustomPropertyDeclaration.h"
 #include "core/css/CSSFontFeatureValue.h"
 #include "core/css/CSSFunctionValue.h"
 #include "core/css/CSSGridLineNamesValue.h"
@@ -1353,6 +1354,11 @@ static PassRefPtrWillBeRawPtr<CSSValue> valueForScrollSnapCoordinate(const Vecto
     return list.release();
 }
 
+PassRefPtrWillBeRawPtr<CSSValue> ComputedStyleCSSValueMapping::get(const AtomicString customPropertyName, const ComputedStyle& style)
+{
+    return CSSCustomPropertyDeclaration::create(customPropertyName, style.variables()->getVariable(customPropertyName));
+}
+
 PassRefPtrWillBeRawPtr<CSSValue> ComputedStyleCSSValueMapping::get(CSSPropertyID propertyID, const ComputedStyle& style, const LayoutObject* layoutObject, Node* styledNode, bool allowVisitedStyle)
 {
     const SVGComputedStyle& svgStyle = style.svgStyle();