Suppress JavaScript Dialogs triggered from different origin subframes

As described in https://www.chromestatus.com/feature/5148698084376576 , JavaScript modal dialogs, triggered by window.alert, window.confirm, and window.prompt, will be blocked in Brave if triggered from a subframe whose origin is different from the main frame origin. This policy allows overriding that change. If the policy is set to enabled or unset, JavaScript dialogs triggered from a different origin subframe will be blocked. If the policy is set to disabled, JavaScript dialogs triggered from a different origin subframe will not be blocked. This policy will be removed from Brave in the future.