Update some comments about variations headers API use.

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 155 matching lines @@
   GURL request_url(BuildRequestUrl());
   DCHECK(request_url.is_valid());
 
   // Reset will delete any previous fetcher, and we won't get any callback.
   search_term_fetcher_ = net::URLFetcher::Create(
       kContextualSearchURLFetcherID, request_url, net::URLFetcher::GET, this);
   search_term_fetcher_->SetRequestContext(browser_state_->GetRequestContext());
 
   // Add Chrome experiment state to the request headers.
   net::HttpRequestHeaders headers;
-  // Note: It's fine to pass in |is_signed_in| false, which does not affect
-  // transmission of experiment ids coming from the variations server.
+  // Note: It's OK to pass |is_signed_in| false if it's unknown, as it does
+  // not affect transmission of experiments coming from the variations server.
   bool is_signed_in = false;
   variations::AppendVariationHeaders(search_term_fetcher_->GetOriginalURL(),
                                      browser_state_->IsOffTheRecord(), false,
                                      is_signed_in, &headers);
   search_term_fetcher_->SetExtraRequestHeaders(headers.ToString());
 
   SetDiscourseContextAndAddToHeader(*context_);
 
   search_term_fetcher_->Start();
 }
@@ skipping 136 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);
 }