[ios] Adds VoiceSearchProvider::IsVoiceSearchEnabled().

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"
 
 #import "ios/public/provider/chrome/browser/voice/voice_search_controller.h"
 
+bool VoiceSearchProvider::IsVoiceSearchEnabled() const {
+  return false;
+}
+
 NSArray* VoiceSearchProvider::GetAvailableLanguages() const {
   return @[];
 }
 
 AudioSessionController* VoiceSearchProvider::GetAudioSessionController() const {
   return nullptr;
 }
 
 scoped_refptr<VoiceSearchController>
 VoiceSearchProvider::CreateVoiceSearchController(
     ios::ChromeBrowserState* browser_state) const {
   return scoped_refptr<VoiceSearchController>(nullptr);
 }
 
 UIView<VoiceSearchBar>* VoiceSearchProvider::CreateVoiceSearchBar(
     CGRect frame) const {
   return nil;
 }