Create a FakeTetherService, which stubs out TetherService for development.

chrome/browser/chromeos/tether/tether_service.cc

Index: chrome/browser/chromeos/tether/tether_service.cc
diff --git a/chrome/browser/chromeos/tether/tether_service.cc b/chrome/browser/chromeos/tether/tether_service.cc
index e0dd94fb4b1fd24d64c816e0c5b27eda2f492663..63df714ed929021af1fd1506b1d105ff2d351882 100644
--- a/chrome/browser/chromeos/tether/tether_service.cc
+++ b/chrome/browser/chromeos/tether/tether_service.cc
@@ -176,6 +176,12 @@ void TetherService::OnPrefsChanged() {
   UpdateTetherTechnologyState();
 }
 
+bool TetherService::HasSyncedTetherHosts() const {
+  return !cryptauth_service_->GetCryptAuthDeviceManager()
+              ->GetTetherHosts()
+              .empty();
+}
+
 void TetherService::UpdateTetherTechnologyState() {
   chromeos::NetworkStateHandler::TechnologyState tether_technology_state =
       GetTetherTechnologyState();
@@ -220,12 +226,6 @@ void TetherService::OnBluetoothAdapterFetched(
   UpdateTetherTechnologyState();
 }
 
-bool TetherService::HasSyncedTetherHosts() const {
-  return !cryptauth_service_->GetCryptAuthDeviceManager()
-              ->GetTetherHosts()
-              .empty();
-}
-
 bool TetherService::IsFeatureFlagEnabled() const {
   return base::CommandLine::ForCurrentProcess()->HasSwitch(
       chromeos::switches::kEnableTether);