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

chrome/browser/instant/search.cc

Index: chrome/browser/instant/search.cc
diff --git a/chrome/browser/instant/search.cc b/chrome/browser/instant/search.cc
index 2816d5250fcd482487b0feb49968e12830d62cdf..9a1c34715b288ba1075611337a894689ce5153f3 100644
--- a/chrome/browser/instant/search.cc
+++ b/chrome/browser/instant/search.cc
@@ -17,6 +17,7 @@
 #include "chrome/browser/search_engines/template_url_service_factory.h"
 #include "chrome/common/chrome_switches.h"
 #include "chrome/common/pref_names.h"
+#include "chrome/common/search_types.h"
 #include "chrome/common/url_constants.h"
 #include "components/user_prefs/pref_registry_syncable.h"
 #include "content/public/browser/navigation_entry.h"
@@ -522,5 +523,16 @@ GURL CoerceCommandLineURLToTemplateURL(const GURL& instant_url,
   return instant_url.ReplaceComponents(replacements);
 }
 
+bool ShouldHandleTopBarsVisibilityChange(const Mode& old_mode,
+                                         const Mode& new_mode) {
+  // If mode has changed, only handle top bars visibility change if new mode
+  // is not |SEARCH_SUGGESTIONS| or |SEARCH_RESULTS|.  Top bars visibility for
+  // these 2 modes is determined when the mode stays the same, and:
+  // - for |NTP/SERP| pages: by SearchBox API, or
+  // - for |DEFAULT| pages: by platform-specific implementation of
+  //   |InstantOverlayController| when it shows/hides the Instant overlay.
+  return old_mode != new_mode ? !new_mode.is_search() : new_mode.is_search();
+}
+
 }  // namespace search
 }  // namespace chrome