[css-typed-om] Replace background-position with object-position (w3c#428)

* Replace background-position with object-position

* Remove issue from spec

Author: darrnshn

css-typed-om/Overview.bs

@@ -2060,9 +2060,7 @@ is a [=list=] of {{CSSTransformComponent}}s.
 --------------------------------------------------
 
 {{CSSPositionValue}} objects represent <<position>> values,
-used by properties such as 'background-position'.
-
-Issue(312): background-position is a shorthand
+used by properties such as 'object-position'.
 
 <pre class='idl'>
 [Constructor(CSSNumericValue x, CSSNumericValue y)]
@@ -2111,7 +2109,7 @@ The {{CSSPositionValue/x}} attribute expresses the offset from the left edge of
 
     <pre class='lang-css'>
     .example {
-        background-position: center bottom 10px;
+        object-position: center bottom 10px;
     }
     </pre>
 
@@ -2120,10 +2118,10 @@ The {{CSSPositionValue/x}} attribute expresses the offset from the left edge of
     <pre class='lang-javascript'>
     let map = document.querySelector('.example').styleMap;
 
-    map.get('background-position').x;
+    map.get('object-position').x;
     // CSS.percent(50)
 
-    map.get('background-position').y;
+    map.get('object-position').y;
     // CSSMathSum(CSS.percent(100), CSS.px(-10))
     </pre>
 </div>