Add heuristics to limit showing of new backspace UI bubble.

chrome/browser/ui/browser_view_prefs.cc

Index: chrome/browser/ui/browser_view_prefs.cc
diff --git a/chrome/browser/ui/browser_view_prefs.cc b/chrome/browser/ui/browser_view_prefs.cc
index 3232859dfc2110e43a4e9da7c8916756ce13bd3b..4d81f5c3b7e6f6827c68bdfbf6cc9f37675382c7 100644
--- a/chrome/browser/ui/browser_view_prefs.cc
+++ b/chrome/browser/ui/browser_view_prefs.cc
@@ -36,9 +36,12 @@ void RegisterBrowserViewProfilePrefs(
 #if defined(OS_LINUX) && !defined(OS_CHROMEOS)
   custom_frame_default = ui::GetCustomFramePrefDefault();
 #endif
-
   registry->RegisterBooleanPref(prefs::kUseCustomChromeFrame,
                                 custom_frame_default);
+
+  registry->RegisterIntegerPref(
+      prefs::kBackShortcutBubbleShownCount, 0,
+      user_prefs::PrefRegistrySyncable::SYNCABLE_PREF);
 }
 
 void MigrateBrowserTabStripPrefs(PrefService* prefs) {