alternate ntp: implement Show/HideBars API to reduce jank when showing/hiding bars

chrome/browser/ui/search/search_model.cc

 // 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.
 
 #include "chrome/browser/ui/search/search_model.h"
 
 #include "chrome/browser/ui/search/search.h"
 #include "chrome/browser/ui/search/search_model_observer.h"
 
 namespace chrome {
 namespace search {
 
-SearchModel::SearchModel() {
+SearchModel::SearchModel() : show_top_bars_(true) {
 }
 
 SearchModel::~SearchModel() {
 }
 
 void SearchModel::SetMode(const Mode& new_mode) {
   DCHECK(IsInstantExtendedAPIEnabled())
       << "Please do not try to set the SearchModel mode without first "
       << "checking if Search is enabled.";
 
   if (mode_ == new_mode)
     return;
 
   const Mode old_mode = mode_;
   mode_ = new_mode;
 
   FOR_EACH_OBSERVER(SearchModelObserver, observers_,
                     ModeChanged(old_mode, mode_));
 }
 
+void SearchModel::SetTopBarsVisible(bool visible) {
+  DCHECK(IsInstantExtendedAPIEnabled())
+      << "Please do not try to set the SearchModel mode without first "
+      << "checking if Search is enabled.";
+
+  show_top_bars_ = visible;
+
+  FOR_EACH_OBSERVER(SearchModelObserver, observers_,
+                    TopBarsVisibilityChanged(mode_, visible));
+}
+
 void SearchModel::AddObserver(SearchModelObserver* observer) {
   observers_.AddObserver(observer);
 }
 
 void SearchModel::RemoveObserver(SearchModelObserver* observer) {
   observers_.RemoveObserver(observer);
 }
 
 } //  namespace search
 } //  namespace chrome