CMS Page - Force validate layout update xml, even in production mode, when saving CMS Page

[adrian-martinez-interactiv4]

When saving custom layout update xml in CMS page, in production mode, xml is not validated against schema file, due to \Magento\Framework\App\Arguments\ValidationState::isValidationRequiredmethod:

    /**
     * Retrieve current validation state
     *
     * @return boolean
     */
    public function isValidationRequired()
    {
        return $this->_appMode == \Magento\Framework\App\State::MODE_DEVELOPER;
    }

Actual result:

Desired behaviour would be check the layout update xml against the schema, and show error message to the user if it is invalid:

Description

• \Magento\Cms\Model\Page\DomValidationState has been added, and it is injected from post data processor to force layout update xml validation, only upon cms page save.

When this PR is applied and validation is performed, application may crush due to unhandled exception. This issue is solved by PR #11857

Manual testing scenarios

1. Enable production mode
2. Edit or create a CMS page, enter some invalid xml in Layout Update XML textarea
3. Try to save the page

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[dmanners]

@adrian-martinez-interactiv4 a couple of Test failures to sort out for us please.

[adrian-martinez-interactiv4]

Changes of this PR included in PR #11857