Make the backdrop fixed positioned, matching the spec.

dialog-polyfill.css

@@ -23,7 +23,7 @@ dialog[open] {
 }
 
 .backdrop {
-  position: absolute;
+  position: fixed;
   background: rgba(0,0,0,0.1);
   top: 0;
   bottom: 0;

tests/backdrop.html

@@ -11,11 +11,6 @@ dialog {
 }
 
 .backdrop {
-  position: fixed;
-  top: 0;
-  bottom: 0;
-  left: 0;
-  right: 0;
   background-color: rgba(0,255,0,0.5);
 }
 </style>