Customizer: Reset horizontal scroll position when finished dragging a nav menu item.

Cherry-picks [34834].

Props karinedo, austinginder, adamsilverstein, tyxla.
Fixes #33367 for 4.3.

Built from https://develop.svn.wordpress.org/branches/4.3@34835


git-svn-id: http://core.svn.wordpress.org/branches/4.3@34800 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/js/customize-nav-menus.js

@@ -1871,6 +1871,9 @@
 
 					control.isSorting = false;
 
+					// Reset horizontal scroll position when done dragging.
+					control.$sectionContent.scrollLeft( 0 );
+
 					_.each( menuItemContainerIds, function( menuItemContainerId ) {
 						var menuItemId, menuItemControl, matches;
 						matches = menuItemContainerId.match( /^customize-control-nav_menu_item-(-?\d+)$/, '' );