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

chrome/browser/ui/search/search_model.cc

Index: chrome/browser/ui/search/search_model.cc
diff --git a/chrome/browser/ui/search/search_model.cc b/chrome/browser/ui/search/search_model.cc
index f38ffad9b97193b48ce2c530193b680e44a02f18..a6c2d91614a4c16f58f4dc2a1f298ef84e81416a 100644
--- a/chrome/browser/ui/search/search_model.cc
+++ b/chrome/browser/ui/search/search_model.cc
@@ -10,7 +10,7 @@
 namespace chrome {
 namespace search {
 
-SearchModel::SearchModel() {
+SearchModel::SearchModel() : show_top_bars_(true) {
 }
 
 SearchModel::~SearchModel() {
@@ -31,6 +31,17 @@ void SearchModel::SetMode(const Mode& new_mode) {
                     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);
 }