BackgroundImage incorrectly returns empty url() when created on-the-fly

Source/core/css/CSSImageSetValue.cpp

Index: Source/core/css/CSSImageSetValue.cpp
diff --git a/Source/core/css/CSSImageSetValue.cpp b/Source/core/css/CSSImageSetValue.cpp
index e02ce18ecc2d60cda0c8f695f0cfebf80eee4ba6..30d12bc15efa4fbea9780acb330a3738bb1d13cc 100644
--- a/Source/core/css/CSSImageSetValue.cpp
+++ b/Source/core/css/CSSImageSetValue.cpp
@@ -188,4 +188,18 @@ DEFINE_TRACE_AFTER_DISPATCH(CSSImageSetValue)
     CSSValueList::traceAfterDispatch(visitor);
 }
 
+PassRefPtrWillBeRawPtr<CSSImageSetValue> CSSImageSetValue::valueWithAbsoluteURL()
+{
+    CSSImageSetValue* value = CSSImageSetValue::create().get();
+    for (auto& item : *this) {
+        if (item->isImageValue()) {
+            value->append(toCSSImageValue(*item).valueWithAbsoluteURL());
+        } else {
+            value->append(item);
+        }
+    }
+    return adoptRef(value);
+}
+
+
 } // namespace blink