Allow ServiceWorker to dispatch navigation requests without waiting for its startup

https://github.com/WICG/service-worker-auto-preload The ServiceWorkerAutoPreload feature dispatches a network request for a main resource at the same time it begins the ServiceWorker bootstrap process. Setting the policy to Enabled or leaving it unset means Brave enables ServiceWorkerAutoPreload. The navigation request is automatically dispatched while starting the ServiceWorker in some scenarios, e.g. ServiceWorker is not running, If it is disabled, Brave will not enable ServiceWorkerAutoPreload. The navigation request is dispatched always after starting the ServiceWorker. This policy is a temporary measure to control the feature and will be removed in M144.