Move background related shorthands into CSSPropertyParser

third_party/WebKit/LayoutTests/fast/css/parsing-unitless-length-quirk.html

 <script src="../../resources/testharness.js"></script>
 <script src="../../resources/testharnessreport.js"></script>
 <style>
 #t1 { border-top-width: 1 }
 #t2 { border-right-width: 1 }
 #t3 { border-bottom-width: 1 }
 #t4 { border-left-width: 1 }
 #t5 { border-width: 1 }
 #t6 { bottom: 1 }
 #t7 { clip: rect(1 1 1 1) }
@@ skipping 165 matching lines @@
 test(function(){
     assert_equals(sheet.cssRules[38].style.border, "");
 }, "No unitless length quirk for border property");
 test(function(){
     assert_equals(sheet.cssRules[39].style.font, "");
 }, "No unitless length quirk for font property");
 test(function(){
     assert_equals(sheet.cssRules[40].style.font, "");
 }, "No unitless length quirk for font property");
 test(function(){
-    assert_equals(sheet.cssRules[41].style.backgroundPosition, "10px 50%");
+    assert_equals(sheet.cssRules[41].style.backgroundPosition, "10px center");
 }, "Unitless length quirk for background-position property");
 test(function(){
     assert_equals(sheet.cssRules[42].style.backgroundPosition, "10px 20px");
 }, "Unitless length quirk for background-position property");
 test(function(){
     assert_equals(sheet.cssRules[43].style.background, "");
 }, "No unitless length quirk for background property");
 test(function(){
     assert_equals(sheet.cssRules[44].style.background, "");
 }, "No unitless length quirk for background property");
@@ skipping 15 matching lines @@
 test(function(){
     assert_equals(sheet.cssRules[50].style.minHeight, "10px");
 }, "Unitless length quirk for min-height property");
 test(function(){
     assert_equals(sheet.cssRules[51].style.maxWidth, "10px");
 }, "Unitless length quirk for max-width property");
 test(function(){
     assert_equals(sheet.cssRules[52].style.maxHeight, "10px");
 }, "Unitless length quirk for max-height property");
 </script>