Autoprefix box sizing

Results are from running grunt autoprefix.  Needed for FF before 29 and Safari before 5.1.

fixes #27553



git-svn-id: https://develop.svn.wordpress.org/trunk@30070 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/css/editor.css

@@ -828,6 +828,8 @@ i.mce-i-hr:before {
 
 .wp-switch-editor {
 	float: left;
+	-webkit-box-sizing: content-box;
+	-moz-box-sizing: content-box;
 	box-sizing: content-box;
 	position: relative;
 	top: 1px;