shill: Support Cellular CDMA activation.

Still TODO is communicating the activation error codes to the Cellular service.
... And, of course, asynchronous calls throughout.

BUG=chromium-os:19305
TEST=unit tests

Change-Id: I3e0b4a5a8d3c4c74db35ce7f74b5d958677d8b66
Reviewed-on: http://gerrit.chromium.org/gerrit/6192
Tested-by: Darin Petkov <petkov@chromium.org>
Reviewed-by: Chris Masone <cmasone@chromium.org>
diff --git a/cellular_service.cc b/cellular_service.cc
index 654c5b4..f0cb2ab 100644
--- a/cellular_service.cc
+++ b/cellular_service.cc
@@ -51,7 +51,11 @@
 
 void CellularService::Disconnect() { }
 
-std::string CellularService::GetDeviceRpcId() {
+void CellularService::ActivateCellularModem(const string &carrier) {
+  cellular_->Activate(carrier);
+}
+
+string CellularService::GetDeviceRpcId() {
   return cellular_->GetRpcIdentifier();
 }