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

ios/chrome/browser/ui/contextual_search/contextual_search_delegate.cc

 // Copyright 2014 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.
 
 #include "ios/chrome/browser/ui/contextual_search/contextual_search_delegate.h"
 
 #include <algorithm>
 #include <utility>
 
 #include "base/base64.h"
@@ skipping 248 matching lines @@
 
   std::string request =
       GetSearchTermResolutionUrlString(selected_text_escaped, base_page_url);
 
   return GURL(request);
 }
 
 std::string ContextualSearchDelegate::GetSearchTermResolutionUrlString(
     const std::string& selected_text,
     const std::string& base_page_url) {
-  TemplateURL* template_url = template_url_service_->GetDefaultSearchProvider();
+  const TemplateURL* template_url =
+      template_url_service_->GetDefaultSearchProvider();
 
   TemplateURLRef::SearchTermsArgs search_terms_args =
       TemplateURLRef::SearchTermsArgs(base::string16());
 
   TemplateURLRef::SearchTermsArgs::ContextualSearchParams params(
       kContextualSearchRequestVersion, kContextualCardsNoIntegration,
       std::string());
 
   search_terms_args.contextual_search_params = params;
 
@@ skipping 45 matching lines @@
   proto.SerializeToString(&serialized);
 
   std::string encoded_context;
   base::Base64Encode(serialized, &encoded_context);
   // The server memoizer expects a web-safe encoding.
   std::replace(encoded_context.begin(), encoded_context.end(), '+', '-');
   std::replace(encoded_context.begin(), encoded_context.end(), '/', '_');
   search_term_fetcher_->AddExtraRequestHeader(kDiscourseContextHeaderPrefix +
                                               encoded_context);
 }