Fix background-size serializing

third_party/WebKit/LayoutTests/fast/css/background-image-size-serialization.html

+<script src="../../resources/js-test.js"></script>
+<style>
+.noposition
+{
+    background: url("foo.png");
+    background-size: contain;
+}
+.position1
+{
+    background: url("foo.png");
+    background-position: 1px;
+    background-size: contain;
+}
+.position2
+{
+    background: url("foo.png");
+    background-position-x: 1px;
+    background-size: contain;
+}
+.position3
+{
+    background: url("foo.png");
+    background-position: 1px 1px;
+    background-size: contain;
+}
+</style>
+<script>
+var cssRules = document.styleSheets[1].cssRules;
+
+shouldBe('cssRules.length', '4');
+shouldBeEqualToString('cssRules[0].cssText', '.noposition { background: url("foo.png") 0% 0% / contain; }');
+shouldBeEqualToString('cssRules[1].cssText', '.position1 { background: url("foo.png") 1px 50% / contain; }');
+shouldBeEqualToString('cssRules[2].cssText', '.position2 { background: url("foo.png") 1px / contain; }');
+shouldBeEqualToString('cssRules[3].cssText', '.position3 { background: url("foo.png") 1px 1px / contain; }');
+</script>