Add heuristics to limit showing of new backspace UI bubble.

chrome/browser/ui/cocoa/browser_window_cocoa.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CHROME_BROWSER_UI_COCOA_BROWSER_WINDOW_COCOA_H_
 #define CHROME_BROWSER_UI_COCOA_BROWSER_WINDOW_COCOA_H_
 
 #include "base/mac/scoped_nsobject.h"
 #include "base/memory/weak_ptr.h"
 #include "chrome/browser/extensions/extension_keybinding_registry.h"
@@ skipping 64 matching lines @@
   gfx::Rect GetBounds() const override;
   gfx::Size GetContentsSize() const override;
   bool IsMaximized() const override;
   bool IsMinimized() const override;
   void Maximize() override;
   void Minimize() override;
   void Restore() override;
   bool ShouldHideUIForFullscreen() const override;
   bool IsFullscreen() const override;
   bool IsFullscreenBubbleVisible() const override;
-  void ShowNewBackShortcutBubble(bool forward) override;
+  void MaybeShowNewBackShortcutBubble(bool forward) override;
+  void HideNewBackShortcutBubble() override;
   LocationBar* GetLocationBar() const override;
   void SetFocusToLocationBar(bool select_all) override;
   void UpdateReloadStopState(bool is_loading, bool force) override;
   void UpdateToolbar(content::WebContents* contents) override;
   void ResetToolbarTabState(content::WebContents* contents) override;
   void FocusToolbar() override;
   ToolbarActionsBar* GetToolbarActionsBar() override;
   void ToolbarSizeChanged(bool is_animating) override;
   void FocusAppMenu() override;
   void FocusBookmarksToolbar() override;
@@ skipping 99 matching lines @@
   ui::WindowShowState initial_show_state_;
   NSInteger attention_request_id_;  // identifier from requestUserAttention
 
   // Preserves window alert state to show appropriate icon in the window title
   // which can be audio playing, muting or none (determined by alert state of
   // tabs.
   TabAlertState alert_state_;
 };
 
 #endif  // CHROME_BROWSER_UI_COCOA_BROWSER_WINDOW_COCOA_H_