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 8a26e926bbe3f41c2f25f4452ed50cadff92df26..4db6c506680ed9e8949f14464edf876cdc9de101 100644
--- a/chrome/browser/ui/cocoa/browser_window_cocoa.mm
+++ b/chrome/browser/ui/cocoa/browser_window_cocoa.mm
@@ -434,10 +434,13 @@ 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 {