[FIX] web_editor: apply color unremovable

Steps to reproduce the issue:
=============================
- Go to website and open editor
- Click on the 'Contact Us' button
- Change the color of the text
- error

Origin of the issue:
====================
`HistoryReverCurrentStep` will call `observerFlush` which will mark
`_toRollback = true` and in `_observeOdooFieldChanges` we will update
the html with `withoutRollback` but this only works only if
`_toRollback` is `false`.

Solution:
=========
We need to rever  the step without rollback too.

task-3770287

closes odoo/odoo#156461

Signed-off-by: David Monjoie (dmo) <dmo@odoo.com>

addons/web_editor/static/src/js/wysiwyg/wysiwyg.js

@@ -190,7 +190,8 @@ export class Wysiwyg extends Component {
             selectedTab: 'theme-colors',
             withGradients: true,
             onColorLeave: () => {
-                this.odooEditor.historyRevertCurrentStep();
+                // We need to prevent rollback in case the seclection is in unremovable
+                this.odooEditor.withoutRollback(() => this.odooEditor.historyRevertCurrentStep());
                 // Compute the selection to ensure it's preserved between
                 // selectionchange events in case this gets triggered multiple
                 // times quickly.