Implement Physical Web listener registration

ios/chrome/common/physical_web/ios_chrome_physical_web_data_source.mm

 // Copyright 2016 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.
 
 #import "ios/chrome/common/physical_web/ios_chrome_physical_web_data_source.h"
 
 #include "base/memory/ptr_util.h"
 #include "base/values.h"
 #import "ios/chrome/common/physical_web/physical_web_scanner.h"
 
@@ skipping 27 matching lines @@
 std::unique_ptr<base::ListValue> IOSChromePhysicalWebDataSource::GetMetadata() {
   if (!scanner_) {
     return base::MakeUnique<base::ListValue>();
   }
   return [scanner_ metadata];
 }
 
 bool IOSChromePhysicalWebDataSource::HasUnresolvedDiscoveries() {
   return [scanner_ unresolvedBeaconsCount] > 0;
 }
-
-void IOSChromePhysicalWebDataSource::RegisterListener(
-    PhysicalWebListener* physical_web_listener) {}
-
-void IOSChromePhysicalWebDataSource::UnregisterListener(
-    PhysicalWebListener* physical_web_listener) {}