[Extensions Menu] Strict-ify DCHECK in ToolbarActionsBar

The ToolbarActionsBar should never be constructed, at all, with the
kExtensionsToolbarMenu feature enabled. Make the DCHECK in the
ToolbarActionsBar constructor stricter to make this clear, and catch any
cases where we erroneously construct one.

Bug: None

Change-Id: I5ddef89f249435b0fcfe97459b5c09bd88936075
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1877449Reviewed-by: Peter Boström <pbos@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709133}

chrome/browser/ui/toolbar/toolbar_actions_bar.cc

@@ -113,8 +113,7 @@ ToolbarActionsBar::ToolbarActionsBar(ToolbarActionsBarDelegate* delegate,
   if (model_)  // |model_| can be null in unittests.
     model_observer_.Add(model_);
 
-  if (base::FeatureList::IsEnabled(features::kExtensionsToolbarMenu))
-    DCHECK(!in_overflow_mode());
+  DCHECK(!base::FeatureList::IsEnabled(features::kExtensionsToolbarMenu));
 
   browser_->tab_strip_model()->AddObserver(this);
 }