Add heuristics to limit showing of new backspace UI bubble.

chrome/browser/ui/cocoa/browser_window_cocoa.mm

Index: chrome/browser/ui/cocoa/browser_window_cocoa.mm
diff --git a/chrome/browser/ui/cocoa/browser_window_cocoa.mm b/chrome/browser/ui/cocoa/browser_window_cocoa.mm
index 14f8b8a98ceb5d9fcddf007ccbcf6753148d9fa1..8a730be6db888ac0a3e42d94037649a505ecbb83 100644
--- a/chrome/browser/ui/cocoa/browser_window_cocoa.mm
+++ b/chrome/browser/ui/cocoa/browser_window_cocoa.mm
@@ -433,10 +433,13 @@ bool BrowserWindowCocoa::IsFullscreenBubbleVisible() const {
   return false;  // Currently only called from toolkit-views website_settings.
 }
 
-void BrowserWindowCocoa::ShowNewBackShortcutBubble(bool forward) {
-  ExclusiveAccessController* exclusive_access_controller =
-      [controller_ exclusiveAccessController];
-  exclusive_access_controller->ShowNewBackShortcutBubble(forward);
+void BrowserWindowCocoa::MaybeShowNewBackShortcutBubble(bool forward) {
+  [controller_ exclusiveAccessController]->MaybeShowNewBackShortcutBubble(
+      forward);
+}
+
+void BrowserWindowCocoa::HideNewBackShortcutBubble() {
+  [controller_ exclusiveAccessController]->HideNewBackShortcutBubble();
 }
 
 LocationBar* BrowserWindowCocoa::GetLocationBar() const {