Add experiment CHECK to ExtensionToolbarMenuView

Makes sure this class is not instantiated during the
kExtensionsToolbarMenu experiment. Also clearly documents that we expect
this class to be removed once the experiment is launched.

Bug: chromium:985834
Change-Id: Ic9cd2bb03cba4a3765317cb6d67bf7a2676e418c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1814661
Auto-Submit: Peter Boström <pbos@chromium.org>
Commit-Queue: Caroline Rising <corising@chromium.org>
Reviewed-by: Caroline Rising <corising@chromium.org>
Cr-Commit-Position: refs/heads/master@{#698659}

chrome/browser/ui/views/toolbar/extension_toolbar_menu_view.cc

@@ -9,6 +9,7 @@
 #include "base/single_thread_task_runner.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "base/time/time.h"
+#include "chrome/browser/ui/ui_features.h"
 #include "chrome/browser/ui/views/frame/browser_view.h"
 #include "chrome/browser/ui/views/toolbar/app_menu.h"
 #include "chrome/browser/ui/views/toolbar/browser_actions_container.h"
@@ -28,6 +29,7 @@ ExtensionToolbarMenuView::ExtensionToolbarMenuView(
     Browser* browser,
     views::MenuItemView* menu_item)
     : browser_(browser), menu_item_(menu_item) {
+  CHECK(!base::FeatureList::IsEnabled(features::kExtensionsToolbarMenu));
   auto* browser_view = BrowserView::GetBrowserViewForBrowser(browser_);
   auto* toolbar_button_provider = browser_view->toolbar_button_provider();
   auto* app_menu_button = toolbar_button_provider->GetAppMenuButton();