Preserve identical values for background-size and -webkit-mask-size.

LayoutTests/fast/backgrounds/size/resources/parsing-background-size-values.js

 description("This tests checks that all of the input values for background-size parse correctly.");
 
 function test(value)
 {
     var div = document.createElement("div");
     div.setAttribute("style", value);
     document.body.appendChild(div);
     
     var result = div.style.getPropertyValue(value.substring(0, value.indexOf(":")));
     document.body.removeChild(div);
     return result;
 }
 
 shouldBe('test("background-size: contain;")', '"contain"');
 shouldBe('test("background-size: cover;")', '"cover"');
-shouldBe('test("background-size: 100 100;")', '"100px"');
-shouldBe('test("background-size: 100px 100px;")', '"100px"');
+shouldBe('test("background-size: 100 100;")', '"100px 100px"');
+shouldBe('test("background-size: 100px 100px;")', '"100px 100px"');
 shouldBe('test("background-size: auto 50px;")', '"auto 50px"');
 shouldBe('test("background-size: 50px auto;")', '"50px"');
 shouldBe('test("background-size: auto auto;")', '"auto"');
 shouldBe('test("background-size: 30% 20%;")', '"30% 20%"');
 shouldBe('test("background-size: 4em auto;")', '"4em"');
 shouldBe('test("background-size: 5em;")', '"5em"');
 shouldBe('test("-webkit-background-size: 5em ;")', '"5em"');
 
 shouldBe('test("background-size: 100 100 100;")', 'null');
 shouldBe('test("background-size: coconut;")', 'null');
 
 shouldBe('test("background-size: 100px,;")', 'null');
 shouldBe('test("background-size: 100px, 50%;")', '"100px, 50%"');
 shouldBe('test("-webkit-background-size: 100px, 50%;")', '"100px, 50%"');
 shouldBe('test("background-size: 50% 100px, 2em, 100px 50%;")', '"50% 100px, 2em, 100px 50%"');
 shouldBe('test("-webkit-background-size: 50% 100px, 2em, 100px 50%;")', '"50% 100px, 2em, 100px 50%"');