Unsaved allowed html tags are lost when adding a plugin

Problem/Motivation

While editiing a text format that uses the limit allowed tags filter, if you add some tags to the allowed html tags then drag a plugin to the toolbar before having saved, you lose your changes.

Steps to reproduce:

1. install Drupal standard
2. edit basic HTML format (admin/config/content/formats/manage/basic_html)
3. add <i> to the allowed html tags field (for example)
4. add title to the <a> allowed attributes
5. drag any button to the toolbar

Expected result: the <i> and the <a title> stays allowed
Current result: the <i> and the <a title> are removed from the allowed tags

Proposed resolution

In core/modules/filter/filter.filter_html.admin.js fix Drupal.behaviors.filterFilterHtmlUpdating._updateAllowedTags so it takes the values from the allowed HTML tags field.

Remaining tasks

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User interface changes

Fixes the bug.

API changes

None.

Data model changes

None.