Implement CSSPaintValue and add a layout test.

third_party/WebKit/Source/core/css/CSSPaintImageGenerator.h

Index: third_party/WebKit/Source/core/css/CSSPaintImageGenerator.h
diff --git a/third_party/WebKit/Source/core/css/CSSPaintImageGenerator.h b/third_party/WebKit/Source/core/css/CSSPaintImageGenerator.h
index 9569f34c5ebf07865f5e660297caaab0bf8eca3e..fea34a51cfe8f95448b7afef56b9cd8447bcd972 100644
--- a/third_party/WebKit/Source/core/css/CSSPaintImageGenerator.h
+++ b/third_party/WebKit/Source/core/css/CSSPaintImageGenerator.h
@@ -12,6 +12,7 @@
 
 namespace blink {
 
+class CSSVariableData;
 class Document;
 class Image;
 class LayoutObject;
@@ -47,7 +48,8 @@ class CORE_EXPORT CSSPaintImageGenerator
   // representing an invalid image if an error occurred.
   virtual PassRefPtr<Image> paint(const LayoutObject&,
                                   const IntSize&,
-                                  float zoom) = 0;
+                                  float zoom,
+                                  const Vector<RefPtr<CSSVariableData>>&) = 0;
 
   virtual const Vector<CSSPropertyID>& nativeInvalidationProperties() const = 0;
   virtual const Vector<AtomicString>& customInvalidationProperties() const = 0;