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

ios/chrome/browser/ui/ntp/google_landing_mediator.mm

 // Copyright 2017 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/ntp/google_landing_mediator.h"
 
 #import "base/ios/weak_nsobject.h"
 #include "base/mac/scoped_nsobject.h"
 #include "base/metrics/user_metrics.h"
 #include "base/metrics/user_metrics_action.h"
@@ skipping 190 matching lines @@
       new NotificationPromoWhatsNew(GetApplicationContext()->GetLocalState()));
   _notification_promo->Init();
   [_consumer setPromoText:[base::SysUTF8ToNSString(
                               _notification_promo->promo_text()) copy]];
   [_consumer setPromoIcon:_notification_promo->icon()];
   [_consumer setPromoCanShow:_notification_promo->CanShow()];
 }
 
 - (void)updateShowLogo {
   BOOL showLogo = NO;
-  TemplateURL* defaultURL = _templateURLService->GetDefaultSearchProvider();
+  const TemplateURL* defaultURL =
+      _templateURLService->GetDefaultSearchProvider();
   if (defaultURL) {
     showLogo =
         defaultURL->GetEngineType(_templateURLService->search_terms_data()) ==
         SEARCH_ENGINE_GOOGLE;
   }
   [self.consumer setLogoIsShowing:showLogo];
 }
 
 + (NSUInteger)maxSitesShown {
   return kMaxNumMostVisitedFavicons;
@@ skipping 161 matching lines @@
 
 - (void)onFakeboxBlur {
   [_focuser onFakeboxBlur];
 }
 
 - (void)onFakeboxAnimationComplete {
   [_focuser onFakeboxAnimationComplete];
 }
 
 @end