[FIX] web_editor: prevent editing document in iframe with cross-origin access issue

- Go to Email Marketing
- Create a new mailing with template having social media links
- Save
- Click on one of the social links
An error is triggered:
"Error: Blocked a frame with origin xxx from accessing a cross-origin frame."

The traceback occurs because we try to change the content of something we
don't have access to, as these sites prevent display in an iframe.

opw-2599540

closes odoo/odoo#79188

X-original-commit: 9702e541
Signed-off-by: Nicolas Lempereur (nle) <nle@odoo.com>