Customize: Prevent scrolling `custom_css` textarea to top when pressing tab.

Props tyxla, sstoqnov for testing, celloexpressions for testing.
See #38667.
Merges [39557] to the 4.7 branch.
Fixes #39134.

Built from https://develop.svn.wordpress.org/branches/4.7@39569


git-svn-id: http://core.svn.wordpress.org/branches/4.7@39509 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/js/customize-controls.js

@@ -5300,7 +5300,7 @@
 				} );
 
 				$textarea.on( 'keydown', function onKeydown( event ) {
-					var selectionStart, selectionEnd, value, scroll, tabKeyCode = 9, escKeyCode = 27;
+					var selectionStart, selectionEnd, value, tabKeyCode = 9, escKeyCode = 27;
 
 					if ( escKeyCode === event.keyCode ) {
 						if ( ! $textarea.data( 'next-tab-blurs' ) ) {
@@ -5325,10 +5325,8 @@
 					value = textarea.value;
 
 					if ( selectionStart >= 0 ) {
-						scroll = $textarea.scrollTop;
 						textarea.value = value.substring( 0, selectionStart ).concat( '\t', value.substring( selectionEnd ) );
 						$textarea.selectionStart = textarea.selectionEnd = selectionStart + 1;
-						textarea.scrollTop = scroll;
 					}
 
 					event.stopPropagation();