Update some comments about variations headers API use.

chrome/browser/android/contextualsearch/contextual_search_delegate.cc

 // Copyright 2015 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 "chrome/browser/android/contextualsearch/contextual_search_delegate.h"
 
 #include <algorithm>
 #include <utility>
 
 #include "base/base64.h"
@@ skipping 118 matching lines @@
   DCHECK(request_url.is_valid());
 
   // Reset will delete any previous fetcher, and we won't get any callback.
   search_term_fetcher_.reset(
       net::URLFetcher::Create(kContextualSearchURLFetcherID, request_url,
                               net::URLFetcher::GET, this).release());
   search_term_fetcher_->SetRequestContext(url_request_context_);
 
   // 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(),
       false,  // Impossible to be incognito at this point.
       false, is_signed_in, &headers);
   search_term_fetcher_->SetExtraRequestHeaders(headers.ToString());
 
   SetDiscourseContextAndAddToHeader(*context_);
 
   search_term_fetcher_->Start();
@@ skipping 457 matching lines @@
     end_offset -= trim;
   }
   if (result_text.length() > end_offset + padding_each_side_pinned) {
     // Trim the end.
     result_text = result_text.substr(0, end_offset + padding_each_side_pinned);
   }
   *start = start_offset;
   *end = end_offset;
   return result_text;
 }