Add heuristics to limit showing of new backspace UI bubble.

chrome/browser/ui/views/new_back_shortcut_bubble.h

Index: chrome/browser/ui/views/new_back_shortcut_bubble.h
diff --git a/chrome/browser/ui/views/new_back_shortcut_bubble.h b/chrome/browser/ui/views/new_back_shortcut_bubble.h
index 38b2e9917ebc23d7f13ab0f037ce012240e159c1..4956eb7adbe194d699928e741f6ada19d68ec36a 100644
--- a/chrome/browser/ui/views/new_back_shortcut_bubble.h
+++ b/chrome/browser/ui/views/new_back_shortcut_bubble.h
@@ -33,12 +33,19 @@ class Widget;
 // 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;