Remove non-const version of GetDefaultSearchProvider() and make all callers call the const version

ios/chrome/browser/ui/contextual_search/touch_to_search_permissions_mediator.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/browser/ui/contextual_search/touch_to_search_permissions_mediator.h"
 
 #import <UIKit/UIKit.h>
 
 #include "base/command_line.h"
 #include "base/logging.h"
@@ skipping 194 matching lines @@
 }
 
 #pragma mark - Private methods
 
 - (BOOL)areContextualSearchQueriesSupported {
   TemplateURLService* templateUrlService =
       ios::TemplateURLServiceFactory::GetForBrowserState(_browserState);
   if (!templateUrlService)
     return NO;
 
-  TemplateURL* defaultURL = templateUrlService->GetDefaultSearchProvider();
+  const TemplateURL* defaultURL =
+      templateUrlService->GetDefaultSearchProvider();
 
   // Contextual search is supported if the template URL has a non-empty
   // contextual search URL.
   return !defaultURL->contextual_search_url().empty();
 }
 
 - (BOOL)isVoiceOverEnabled {
   return UIAccessibilityIsVoiceOverRunning();
 }
 
@@ skipping 72 matching lines @@
 
 #pragma mark Notification Center handler
 
 - (void)statusMayHaveChangedWithNotification:(NSNotification*)notification {
   // VoiceOver may have been enabled or disabled, so (if there is an audience
   // object), notify it asynchronously.
   [self maybeNotifyAudienceAsynchronously];
 }
 
 @end