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

components/proximity_auth/connection_observer.h

Index: components/proximity_auth/connection_observer.h
diff --git a/components/proximity_auth/connection_observer.h b/components/proximity_auth/connection_observer.h
index 53578901a29279ca8795db00e88552fb21cf547a..89bf9741c7056e62f0a0ff9f9ab42d70b8a85b11 100644
--- a/components/proximity_auth/connection_observer.h
+++ b/components/proximity_auth/connection_observer.h
@@ -18,18 +18,18 @@
   // |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