Assume all providers may give zero suggest responses.

chrome/browser/autocomplete/history_url_provider.h

 // Copyright (c) 2012 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.
 
 #ifndef CHROME_BROWSER_AUTOCOMPLETE_HISTORY_URL_PROVIDER_H_
 #define CHROME_BROWSER_AUTOCOMPLETE_HISTORY_URL_PROVIDER_H_
 
 #include <string>
 #include <vector>
 
@@ skipping 176 matching lines @@
   // can insert results in appropriate ranges relative to these.
   static const int kScoreForBestInlineableResult;
   static const int kScoreForUnvisitedIntranetResult;
   static const int kScoreForWhatYouTypedResult;
   static const int kBaseScoreForNonInlineableResult;
 
   HistoryURLProvider(AutocompleteProviderListener* listener, Profile* profile);
 
   // HistoryProvider:
   virtual void Start(const AutocompleteInput& input,
-                     bool minimal_changes) override;
+                     bool minimal_changes,
+                     bool on_focus) override;
   virtual void Stop(bool clear_cached_results) override;
 
   // Returns a match representing a navigation to |destination_url| given user
   // input of |text|.  |trim_http| controls whether the match's |fill_into_edit|
   // and |contents| should have any HTTP scheme stripped off, and should not be
   // set to true if |text| contains an http prefix.
   // NOTES: This does not set the relevance of the returned match, as different
   //        callers want different behavior. Callers must set this manually.
   //        This function should only be called on the UI thread.
   AutocompleteMatch SuggestExactInput(const base::string16& text,
@@ skipping 123 matching lines @@
   // keep this member is so we can set the cancel bit on it.
   HistoryURLProviderParams* params_;
 
   // Params controlling experimental behavior of this provider.
   HUPScoringParams scoring_params_;
 
   DISALLOW_COPY_AND_ASSIGN(HistoryURLProvider);
 };
 
 #endif  // CHROME_BROWSER_AUTOCOMPLETE_HISTORY_URL_PROVIDER_H_