Handle initial values in background-repeat introduced by the background shorthand

LayoutTests/fast/css/background-repeat-serialize.html

Index: LayoutTests/fast/css/background-repeat-serialize.html
diff --git a/LayoutTests/fast/css/background-repeat-serialize.html b/LayoutTests/fast/css/background-repeat-serialize.html
index a1a4ee265961d8e8ebef7852d45344a7d4316a3b..f8056204e15840240eaf7959b158a3061e40f025 100644
--- a/LayoutTests/fast/css/background-repeat-serialize.html
+++ b/LayoutTests/fast/css/background-repeat-serialize.html
@@ -34,6 +34,13 @@ test(function() {
   // FIXME (crbug.com/376179): Make setting background-repeat-x/y to multiple values work on CSSStyleDeclarations.
   // assert_equals(serialize('background-repeat-x: repeat, no-repeat; background-repeat-y: no-repeat, repeat, no-repeat'),
   //   'repeat-x, repeat-y, repeat-x, no-repeat, repeat, no-repeat');
-  assert_equals(serialize('background-repeat: repeat, no-repeat, repeat; background-repeat-y: no-repeat'), 'repeat-x, no-repeat, repeat-x');
+  assert_equals(serialize('background-repeat: repeat, no-repeat, repeat; background-repeat-y: no-repeat;'), 'repeat-x, no-repeat, repeat-x');
 }, 'Mismatched value lengths should repeat to their lowest common multiple');
+
+test(function() {
+  assert_equals(serialize('background: url(#1), url(#2), url(#3);'), 'repeat, repeat, repeat');
+  assert_equals(serialize('background: repeat-x, repeat-y, url(#);'), 'repeat-x, repeat-y, repeat');
+  assert_equals(serialize('background: url(#), no-repeat; background-repeat-x: no-repeat'), 'repeat-y, no-repeat');
+  assert_equals(serialize('background: url(#), no-repeat; background-repeat-y: no-repeat'), 'repeat-x, no-repeat');
+}, 'Initial values introduced by the background shorthand should be handled as repeat.');
 </script>