Restrict transmission of external exp ids to signed in users.

components/ntp_snippets/remote/ntp_snippets_fetcher.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_fetcher.h"
 
 #include <cstdlib>
 #include <utility>
 
 #include "base/command_line.h"
@@ skipping 660 matching lines @@
                                    /*default_value=*/false);
 }
 
 std::string NTPSnippetsFetcher::RequestBuilder::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.
+  bool is_signed_in = false;
   variations::AppendVariationHeaders(url_,
                                      false,  // incognito
                                      false,  // uma_enabled
-                                     &headers);
+                                     is_signed_in, &headers);
   return headers.ToString();
 }
 
 std::string NTPSnippetsFetcher::RequestBuilder::BuildBody() const {
   auto request = base::MakeUnique<base::DictionaryValue>();
   std::string user_locale = PosixLocaleFromBCP47Language(params_.language_code);
   switch (fetch_api_) {
     case NTPSnippetsFetcher::CHROME_READER_API: {
       auto content_params = base::MakeUnique<base::DictionaryValue>();
       content_params->SetBoolean("only_return_personalized_results",
@@ skipping 402 matching lines @@
   NOTREACHED();
   return false;
 }
 
 bool NTPSnippetsFetcher::NeedsAuthentication() const {
   return (personalization_ == Personalization::kPersonal ||
           personalization_ == Personalization::kBoth);
 }
 
 }  // namespace ntp_snippets