shelf: Remove undoable TODO.

Notify click wont work because it fires after the release event. We need
to observe both press and release events to mimic keyboard press and
release.

Test: none
Bug: none
Change-Id: I314aed2dcc870c8579a48c8abed45b0065737819
Reviewed-on: https://chromium-review.googlesource.com/954048Reviewed-by: Michael Wasserman <msw@chromium.org>
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541723}

ash/shelf/back_button.h

@@ -27,8 +27,6 @@ class ASH_EXPORT BackButton : public views::ImageButton {
 
  protected:
   // views::ImageButton:
-  // TODO(sammiequon): Consider using NotifyClick instead of OnGesture and
-  // OnMouse.
   void OnGestureEvent(ui::GestureEvent* event) override;
   bool OnMousePressed(const ui::MouseEvent& event) override;
   void OnMouseReleased(const ui::MouseEvent& event) override;