Allow ServiceWorker to control srcdoc iframes

https://github.com/w3c/ServiceWorker/issues/765 asks srcdoc iframe with the "allow-same-origin" sandbox attribute to be under ServiceWorker control. Setting the policy to Enabled or leaving it unset means Brave makes srcdoc iframes with "allow-same-origin" sandbox attributes to be under ServiceWorker control. Setting the policy to Disabled leaves the srcdoc iframe not controlled by ServiceWorker. This policy is intended to be temporary and will be removed in 2026.