[ios] Adds a VoiceSearchController interface to the provider layer.

ios/public/provider/chrome/browser/voice/voice_search_provider.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/public/provider/chrome/browser/voice/voice_search_provider.h"
 
 NSArray* VoiceSearchProvider::GetAvailableLanguages() const {
   return @[];
 }
 
 AudioSessionController* VoiceSearchProvider::GetAudioSessionController() const {
   return nil;
 }
+
+VoiceSearchController* VoiceSearchProvider::CreateVoiceSearchController(
+    ios::ChromeBrowserState* browser_state) const {
+  return nil;
+}