Revert of Revert of [Easy Unlock] Port the BluetoothConnectionFinder class to native code.

components/proximity_auth/connection_observer.h

 // Copyright 2014 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 COMPONENTS_PROXIMITY_AUTH_CONNECTION_OBSERVER_H
 #define COMPONENTS_PROXIMITY_AUTH_CONNECTION_OBSERVER_H
 
 #include "components/proximity_auth/connection.h"
 
 namespace proximity_auth {
 
 class WireMessage;
 
 // An interface for observing events that happen on a Connection.
 class ConnectionObserver {
  public:
   // Called when the |connection|'s status changes from |old_status| to
   // |new_status|.
   virtual void OnConnectionStatusChanged(const Connection& connection,
                                          Connection::Status old_status,
-                                         Connection::Status new_status) = 0;
+                                         Connection::Status new_status) {}
 
   // Called when a |message| is received from a remote device over the
   // |connection|.
   virtual void OnMessageReceived(const Connection& connection,
-                                 const WireMessage& message) = 0;
+                                 const WireMessage& message) {}
 
   // Called after a |message| is sent to the remote device over the
   // |connection|. |success| is |true| iff the message is sent successfully.
   virtual void OnSendCompleted(const Connection& connection,
                                const WireMessage& message,
-                               bool success) = 0;
+                               bool success) {}
 };
 
 }  // namespace proximity_auth
 
 #endif  // COMPONENTS_PROXIMITY_AUTH_CONNECTION_OBSERVER_H