Implement CSSPaintValue and add a layout test.

third_party/WebKit/Source/modules/csspaint/CSSPaintDefinition.h

Index: third_party/WebKit/Source/modules/csspaint/CSSPaintDefinition.h
diff --git a/third_party/WebKit/Source/modules/csspaint/CSSPaintDefinition.h b/third_party/WebKit/Source/modules/csspaint/CSSPaintDefinition.h
index 47590a6c9049ffb3ca42701b876452c95b8bd1bc..c086099ec22ec90fcf7e455f9403b7c54247be57 100644
--- a/third_party/WebKit/Source/modules/csspaint/CSSPaintDefinition.h
+++ b/third_party/WebKit/Source/modules/csspaint/CSSPaintDefinition.h
@@ -14,6 +14,7 @@
 
 namespace blink {
 
+class CSSVariableData;
 class Image;
 class LayoutObject;
 class ScriptState;
@@ -40,7 +41,10 @@ class CSSPaintDefinition final
   //
   // This may return a nullptr (representing an invalid image) if javascript
   // throws an error.
-  PassRefPtr<Image> paint(const LayoutObject&, const IntSize&, float zoom);
+  PassRefPtr<Image> paint(const LayoutObject&,
+                          const IntSize&,
+                          float zoom,
+                          const Vector<RefPtr<CSSVariableData>>&);
   const Vector<CSSPropertyID>& nativeInvalidationProperties() const {
     return m_nativeInvalidationProperties;
   }