Add heuristics to limit showing of new backspace UI bubble.

chrome/browser/ui/views/new_back_shortcut_bubble.h

 // Copyright 2016 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_VIEWS_NEW_BACK_SHORTCUT_BUBBLE_H_
 #define CHROME_BROWSER_UI_VIEWS_NEW_BACK_SHORTCUT_BUBBLE_H_
 
 #include <memory>
 
 #include "base/compiler_specific.h"
@@ skipping 15 matching lines @@
 class Widget;
 }
 
 // NewBackShortcutBubble shows a short-lived notification along the top of the
 // screen when the user presses the old Back/Forward shortcut, telling them how
 // to use the new shortcut. This will only be available for a few milestones to
 // let users adapt.
 // TODO(mgiuca): Remove this in M54 (https://crbug.com/610039).
 class NewBackShortcutBubble : public gfx::AnimationDelegate {
  public:
-  NewBackShortcutBubble(ExclusiveAccessBubbleViewsContext* context,
-                        bool forward);
+  explicit NewBackShortcutBubble(ExclusiveAccessBubbleViewsContext* context);
   ~NewBackShortcutBubble() override;
 
+  // Returns whether the UI is currently visible.
+  bool IsVisible() const;
+
+  // Ensures the UI is displaying the correct shortcut for forward/back based on
+  // |forward|, and resets the hide timer.
   void UpdateContent(bool forward);
 
+  // Fades out the UI immediately.
+  void Hide();
+
  private:
   // gfx::AnimationDelegate:
   void AnimationProgressed(const gfx::Animation* animation) override;
   void AnimationEnded(const gfx::Animation* animation) override;
 
   gfx::Rect GetPopupRect(bool ignore_animation_state) const;
   void OnTimerElapsed();
 
   ExclusiveAccessBubbleViewsContext* const bubble_view_context_;
 
   // Animation controlling showing/hiding of the bubble.
   std::unique_ptr<gfx::SlideAnimation> animation_;
 
   // Timer before the bubble disappears.
   base::OneShotTimer hide_timeout_;
 
   // The contents of the popup.
   SubtleNotificationView* const view_;
 
   views::Widget* const popup_;
 
   DISALLOW_COPY_AND_ASSIGN(NewBackShortcutBubble);
 };
 
 #endif  // CHROME_BROWSER_UI_VIEWS_NEW_BACK_SHORTCUT_BUBBLE_H_