Update some comments about variations headers API use.

components/ntp_snippets/remote/ntp_snippets_json_request.cc

 // 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.
 
 #include "components/ntp_snippets/remote/ntp_snippets_json_request.h"
 
 #include <algorithm>
 #include <utility>
 #include <vector>
 
@@ skipping 338 matching lines @@
   return *this;
 }
 
 std::string NTPSnippetsJsonRequest::Builder::BuildHeaders() const {
   net::HttpRequestHeaders headers;
   headers.SetHeader("Content-Type", "application/json; charset=UTF-8");
   if (!auth_header_.empty()) {
     headers.SetHeader("Authorization", auth_header_);
   }
   // Add X-Client-Data header with experiment IDs from field trials.
-  // 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(url_,
                                      false,  // incognito
                                      false,  // uma_enabled
                                      is_signed_in, &headers);
   return headers.ToString();
 }
 
 std::string NTPSnippetsJsonRequest::Builder::BuildBody() const {
   auto request = base::MakeUnique<base::DictionaryValue>();
@@ skipping 144 matching lines @@
           "NewTabPage.Languages.UILanguageRatioInTwoTopLanguages",
           ratio_ui_in_both_languages * 100);
       break;
     }
   }
 }
 
 }  // namespace internal
 
 }  // namespace ntp_snippets