Add DefaultNetworkChanged signal for ConnectivityStateHelper observers

Add DefaultNetworkChanged signal for ConnectivityStateHelper observers

NetworkLibrary's OnNetworkManagerChanged signal is generated even for
changes on the default network (without a corresponding change on
the NetworkManager). Many consumers (e.g. NetworkScreen) rely on this
behavior to not miss connectivity state updates.

Add an explicit DefaultNetworkChanged signal for ConnectivityStateHelper
observers to get signaled for such events when using the NetworkStateHandler
based implementation (which is the default now).

BUG=236417
TEST=1) Start in OOBE state (no Shill default profile, no Chrome local state).
     2) Observer "Continue" button is grayed out.
     3) Choose a wireless network (say "GoogleGuest")
     4) "Continue" button should be come enabled now.

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=197475

[gauravsh, 2013-04-30 02:15:35]

stevenjb: For ConnectivityStateHelper changes
ygorshenin: For network_screen*.* changes

An alternative though equivalent implementation is to have
ConnectivityStateHelper::NetworkManagerChanged() also be triggered via
DefaultNetworkChange events from NetworkStateHandler. The diff would be smaller.

However, I think we want to keep the ConnectivityStateHelperObserver interface
similar to NetworkStateHandlerObserver interface, so it's easy to get rid of
that layer completely once we are confident in the new implementation.

[ygorshenin1, 2013-04-30 07:43:00]

lgtm

[stevenjb, 2013-04-30 18:35:09]

lgtm

[commit-bot: I haz the power, 2013-04-30 18:40:05]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/gauravsh@chromium.org/14587002/1

[commit-bot: I haz the power, 2013-04-30 21:23:45]

Change committed as 197475