When "sandboxed" is passed as a web preference for `BrowserWindow`, the newly
created renderer won't run any node.js code/integration, only communicating with
the system via the IPC API of the content module. This is a requirement for
running the renderer under chrome OS-level sandbox.
Beyond that, certain behaviors of AtomBrowserClient are modified when dealing
with sandboxed renderers:
- `OverrideSiteInstanceNavigation` no longer create a new `SiteInstance` for
every navigation. Instead, it reuses the source `SiteInstance` when not
navigating to a different site.
- `CanCreateWindow` will return true and allow javascript access.
