Create a FakeTetherService, which stubs out TetherService for development.

chrome/browser/chromeos/tether/fake_tether_service.h

Index: chrome/browser/chromeos/tether/fake_tether_service.h
diff --git a/chrome/browser/chromeos/tether/fake_tether_service.h b/chrome/browser/chromeos/tether/fake_tether_service.h
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+++ b/chrome/browser/chromeos/tether/fake_tether_service.h
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+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef CHROME_BROWSER_CHROMEOS_TETHER_FAKE_TETHER_SERVICE_H_
+#define CHROME_BROWSER_CHROMEOS_TETHER_FAKE_TETHER_SERVICE_H_
+
+#include "chrome/browser/chromeos/tether/tether_service.h"
+
+// A stub of TetherService that provides an easy way to develop for Tether on
+// non-Chromebooks or without a Tether host. To use, see
+// chromeos::switches::kTetherStub for more details.
+class FakeTetherService : public TetherService {
+ public:
+  FakeTetherService(Profile* profile,
+                    chromeos::PowerManagerClient* power_manager_client,
+                    chromeos::SessionManagerClient* session_manager_client,
+                    cryptauth::CryptAuthService* cryptauth_service,
+                    chromeos::NetworkStateHandler* network_state_handler);
+
+  // TetherService:
+  void StartTetherIfEnabled() override;
+  void StopTether() override;
+
+  void set_num_tether_networks(int num_tether_networks) {
+    num_tether_networks_ = num_tether_networks;
+  }
+
+ protected:
+  // TetherService:
+  bool HasSyncedTetherHosts() const override;
+
+ private:
+  int num_tether_networks_ = 1;
+
+  DISALLOW_COPY_AND_ASSIGN(FakeTetherService);
+};
+
+#endif  // CHROME_BROWSER_CHROMEOS_TETHER_FAKE_TETHER_SERVICE_H_