Enhancements for the embedded browser component in AboutWelcome:
- Improve the speed at which the browser starts by predicting the `remoteType` the load is going to finish in
- Set `maychangeremoteness` attribute to true to allow changing remoteness if user navigates to a different URL that requires this (such as an `about:` page, though with the current set up `about:` pages won't load for security reasons).
- Set `nodefaultsrc` attribute to true to avoid loading a default URL if `src` is blank
Differential Revision: https://phabricator.services.mozilla.com/D232425
81a9ec1500