Small bit of tech debt. Currently, when whisking the user away to the NTP, the Welcome UI invokes the Navigate method, rather than the more specific LoadUrl.

per feedback from pmonette@:

> The chrome::Navigate() function is the generalized version where you don't have access to a WebContents. Internally, it will call LoadURL from the controller.
> 
> Also, using chrome::Navigate() requires an inefficient call to GetBrowserForWebContents() (loops over all WebContents of the browser process).
> 
> IMO we should change the navigate in welcome_ui.cc too.