[FIX] web_editor: html_frame: saving in readonly

A form view record may be saved even if the record is displayed in
readonly (e.g. when a button in the form view is clicked). When
this happened, if there were an html field with html_frame widget
in the form, it crashed (e.g. in Email Marketing > Mass Mailings >
open one > click on Test Mailing).

addons/web_editor/static/src/js/backend.js

@@ -383,7 +383,7 @@ var FieldTextHtml = AbstractField.extend({
      * @override
      */
     commitChanges: function () {
-        if (!this.loaded) {
+        if (!this.loaded || this.mode === 'readonly') {
             return;
         }
         // switch to WYSIWYG mode if currently in code mode to get all changes