Remove TODOs to remove the "new back shortcut 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"
 #include "base/macros.h"
 #include "base/timer/timer.h"
 #include "ui/gfx/animation/animation_delegate.h"
 
 class ExclusiveAccessBubbleViewsContext;
 class SubtleNotificationView;
 
 namespace gfx {
 class Animation;
 class Rect;
 class SlideAnimation;
 }
 
 namespace views {
 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).
+// to use the new shortcut. This notification only shows up the first few times
+// the user presses the old shortcut.
 class NewBackShortcutBubble : public gfx::AnimationDelegate {
  public:
   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.
@@ skipping 20 matching lines @@
 
   // 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_