style: Add offset shorthand.

Also, update the serialization by the shorter perference because this is
a new feature and using older syntax doesn't make sense.

Besides, use `cssOffset` for web animation IDL attribute name.

Differential Revision: https://phabricator.services.mozilla.com/D45607

components/style/properties/counted_unknown_properties.py

@@ -9,7 +9,8 @@
 #    "column-span",
 #    "offset-distance",
 #    "offset-path",
-#    "offset-rotate"
+#    "offset-rotate",
+#    "offset"
 COUNTED_UNKNOWN_PROPERTIES = [
     "-webkit-font-smoothing",
     "zoom",
@@ -82,7 +83,6 @@ COUNTED_UNKNOWN_PROPERTIES = [
     "-webkit-ruby-position",
     "-webkit-column-break-after",
     "-webkit-margin-collapse",
-    "offset",
     "-webkit-border-before",
     "-webkit-border-end",
     "-webkit-border-after",