Control zero suggest with a pref, not a switch.

chrome/browser/autocomplete/zero_suggest_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.
 //
 // This file contains the zero-suggest autocomplete provider. This experimental
 // provider is invoked when the user focuses in the omnibox prior to editing,
 // and generates search query suggestions based on the current URL. To enable
 // this provider, point --experimental-zero-suggest-url-prefix at an
 // appropriate suggestion service.
 //
 // HUGE DISCLAIMER: This is just here for experimenting and will probably be
 // deleted entirely as we revise how suggestions work with the omnibox.
 
 #ifndef CHROME_BROWSER_AUTOCOMPLETE_ZERO_SUGGEST_PROVIDER_H_
 #define CHROME_BROWSER_AUTOCOMPLETE_ZERO_SUGGEST_PROVIDER_H_
 
 #include <string>
 #include <vector>
 
 #include "base/basictypes.h"
 #include "base/compiler_specific.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/string16.h"
 #include "chrome/browser/autocomplete/autocomplete_provider.h"
 #include "net/url_request/url_fetcher_delegate.h"
 
 class AutocompleteInput;
 class GURL;
+class PrefService;
 class TemplateURLService;
 
 namespace base {
 class Value;
 }
 
 namespace net {
 class URLFetcher;
 }
 
 // Autocomplete provider for searches based on the current URL.
 //
 // The controller will call StartZeroSuggest when the user focuses in the
 // omnibox. After construction, the autocomplete controller repeatedly calls
 // Start() with some user input, each time expecting to receive an updated
 // set of matches.
 //
 // TODO(jered): Consider deleting this class and building this functionality
 // into SearchProvider after dogfood and after we break the association between
 // omnibox text and suggestions.
 class ZeroSuggestProvider : public AutocompleteProvider,
                             public net::URLFetcherDelegate {
  public:
-  ZeroSuggestProvider(AutocompleteProviderListener* listener,
-                      Profile* profile,
-                      const std::string& url_prefix);
+  // Creates and returns an instance of this provider if the feature is enabled.
+  // Returns NULL if not enabled.
+  static ZeroSuggestProvider* Create(AutocompleteProviderListener* listener,
+                                     Profile* profile);
+
+  static void RegisterUserPrefs(PrefService* user_prefs);
 
   // AutocompleteProvider:
   virtual void Start(const AutocompleteInput& input,
                      bool /*minimal_changes*/) OVERRIDE;
   virtual void Stop(bool clear_cached_results) OVERRIDE;
 
   // net::URLFetcherDelegate
   virtual void OnURLFetchComplete(const net::URLFetcher* source) OVERRIDE;
 
   // Initiates a new fetch for the given |url|, limiting suggestions to those
   // matching |user_text|. |user_text| may be non-empty if the user previously
   // interacted with zero-suggest suggestions and then unfocused the omnibox.
   // TODO(jered): Rip out |user_text| once the first match is decoupled from
   // the current typing in the omnibox.
   void StartZeroSuggest(const GURL& url, const string16& user_text);
 
  private:
+  ZeroSuggestProvider(AutocompleteProviderListener* listener,
+                      Profile* profile,
+                      const std::string& url_prefix);
+
   virtual ~ZeroSuggestProvider();
 
   // Update matches given the user has typed |user_text|.
   void UpdateMatches(const string16& user_text);
 
   // Fetches zero-suggest suggestions for |current_query_|.
   void Run();
 
   // Parses results from the zero-suggest server and updates results.
   // Returns true if results were updated.
@@ skipping 29 matching lines @@
   // Fetcher used to retrieve results.
   scoped_ptr<net::URLFetcher> fetcher_;
 
   // Suggestions for the most recent query.
   std::vector<string16> results_;
 
   DISALLOW_COPY_AND_ASSIGN(ZeroSuggestProvider);
 };
 
 #endif  // CHROME_BROWSER_AUTOCOMPLETE_ZERO_SUGGEST_PROVIDER_H_