Re-add a "private" declaration that was removed unintentionally.

Bug: none
Change-Id: I970d78bdcece269f0d62895df79b7455399676a2
Tbr: rdevlin.cronin
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2468700Reviewed-by: Peter Kasting <pkasting@chromium.org>
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816725}

chrome/browser/ui/views/extensions/extensions_menu_view.h

@@ -103,6 +103,7 @@ class ExtensionsMenuView : public views::BubbleDialogDelegateView,
   // the view directly is more friendly to unit test setups.
   static base::AutoReset<bool> AllowInstancesForTesting();
 
+ private:
   // A "section" within the menu, based on the extension's current access to
   // the page.
   struct Section {