Change ExternalApplicationRegistrar::Register API

Change ExternalApplicationRegistrar::Register API

The initial implementation of ExternalApplicationRegistrar::Register
took a Shell& that couldn't be used until the Register call was
acked. This also meant that the Shell passed to Register() had to be
bound to a Pipe at or before the point-of-call, on the thread where
the call was made. This is problematic for handing this Shell off to a
new application that's going to want to spin up its own main thread.

Since the Shell can't be used at the point of call anyway, this change
alters the Register() API to simply pass back a Shell via a callback,
instead of taking a Shell&.

BUG=407785
TEST=unit tests
R=davemoore

[Chris Masone, 2014-10-03 22:47:10]

Here's the Register API changed and the tests fixed, if you want to use it.