Media: Remove redundant code.

Make the composite gallery library use `mirror` instead of `observe`, as one method now invokes the other. Also, make `mirror` return the attachment collection for chaining.

see #21390.


git-svn-id: https://develop.svn.wordpress.org/trunk@22750 602fd350-edb4-49c9-b593-d223f7449a82

wp-includes/js/media-models.js

@@ -434,6 +434,8 @@ window.wp = window.wp || {};
 			// when `observe()` calls `validateAll()`.
 			this.reset( [], { silent: true } );
 			this.observe( attachments );
+
+			return this;
 		},
 
 		unmirror: function() {

wp-includes/js/media-views.js

@@ -403,12 +403,7 @@
 				return !! original.getByCid( attachment.cid ) && ! exclude.getByCid( attachment.cid );
 			};
 
-			composite.observe( original ).observe( exclude );
-
-			// When `more()` is triggered on the composite collection,
-			// pass the command over to the `original`, which will
-			// populate the query.
-			composite.more = _.bind( original.more, original );
+			composite.mirror( original ).observe( exclude );
 
 			this.set( 'library', composite );
 		},