Remove TODO to make PageActionIconView inherit from Button

This CL removes a TODO for making PageActionIconView inherit from Button.
It is no longer needed because IconLabelBubbleView inherits from Button.

Change-Id: I8e92c6d3cf43e235168d676361264ad1971e9488
Reviewed-on: https://chromium-review.googlesource.com/c/1258669Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Alan Cutter <alancutter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596452}

chrome/browser/ui/views/page_action/page_action_icon_view.h

@@ -33,7 +33,6 @@ class BubbleDialogDelegateView;
 
 // Represents an inbuilt (as opposed to an extension) page action icon that
 // shows a bubble when clicked.
-// TODO(ellyjones): Convert this to subclass Button.
 class PageActionIconView : public IconLabelBubbleView {
  public:
   class Delegate {