[CrOS Tether] Fix build breakage.

chromeos/components/tether/host_scan_cache.h

 // 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 CHROMEOS_COMPONENTS_TETHER_HOST_SCAN_CACHE_H_
 #define CHROMEOS_COMPONENTS_TETHER_HOST_SCAN_CACHE_H_
 
 #include <memory>
 #include <unordered_map>
 
@@ skipping 60 matching lines @@
   // Removes all scan results from the cache unless they correspond to the
   // active host; the active host must always remain in the cache while
   // connecting/connected to ensure the UI is up to date.
   virtual void ClearCacheExceptForActiveHost();
 
   // TetherHostResponseRecorder::Observer:
   void OnPreviouslyConnectedHostIdsChanged() override;
 
   class TimerFactory {
    public:
-    virtual std::unique_ptr<base::Timer> CreateOneShotTimer();
+    virtual std::unique_ptr<base::Timer> CreateOneShotTimer() = 0;
   };
 
  private:
   friend class HostScanCacheTest;
 
   void SetTimerFactoryForTest(
       std::unique_ptr<TimerFactory> timer_factory_for_test);
 
   bool HasConnectedToHost(const std::string& tether_network_guid);
   void StartTimer(const std::string& tether_network_guid);
@@ skipping 14 matching lines @@
   base::WeakPtrFactory<HostScanCache> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(HostScanCache);
 };
 
 }  // namespace tether
 
 }  // namespace chromeos
 
 #endif  // CHROMEOS_COMPONENTS_TETHER_HOST_SCAN_CACHE_H_