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

chrome/browser/ui/browser_instant_controller.cc

Index: chrome/browser/ui/browser_instant_controller.cc
diff --git a/chrome/browser/ui/browser_instant_controller.cc b/chrome/browser/ui/browser_instant_controller.cc
index 5ffb7f048551ff905f7e44d458e205b7f3f3f9ef..fc2d48840fcd42ff0d50ed0353740401e28c5301 100644
--- a/chrome/browser/ui/browser_instant_controller.cc
+++ b/chrome/browser/ui/browser_instant_controller.cc
@@ -236,8 +236,12 @@ void BrowserInstantController::ResetInstant(const std::string& pref_name) {
 ////////////////////////////////////////////////////////////////////////////////
 // BrowserInstantController, search::SearchModelObserver implementation:
 
-void BrowserInstantController::ModeChanged(const search::Mode& old_mode,
-                                           const search::Mode& new_mode) {
+void BrowserInstantController::ModelChanged(const search::Mode& old_mode,
+                                            const search::Mode& new_mode,
+                                            bool top_bars_visible) {
+  if (old_mode == new_mode)
+    return;
+  
   if (search::IsInstantExtendedAPIEnabled()) {
     // Record some actions corresponding to the mode change. Note that to get
     // the full story, it's necessary to look at other UMA actions as well,