Extensions: CodeHealth: Prefer CHECK_IS_TEST() for test-only methods

Reference:
https://chromium.googlesource.com/chromium/src/+/main/styleguide/c++/c++.md#test_only-code-paths-in-production-code

This CL was uploaded by git cl split.

R=finnur@chromium.org

Bug: chromium:346952289
Change-Id: I131285bf93e2498a319ec58df6e37886b6d70689
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6477825
Auto-Submit: Solomon Kinard <solomonkinard@chromium.org>
Reviewed-by: Finnur Thorarinsson <finnur@chromium.org>
Commit-Queue: Finnur Thorarinsson <finnur@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1450187}
diff --git a/chrome/browser/ui/extensions/controlled_home_bubble_delegate.h b/chrome/browser/ui/extensions/controlled_home_bubble_delegate.h
index 6c5fb70..c51a188b 100644
--- a/chrome/browser/ui/extensions/controlled_home_bubble_delegate.h
+++ b/chrome/browser/ui/extensions/controlled_home_bubble_delegate.h
@@ -8,6 +8,7 @@
 #include <optional>
 
 #include "base/auto_reset.h"
+#include "base/check_is_test.h"
 #include "base/scoped_observation.h"
 #include "chrome/browser/ui/toolbar/toolbar_actions_bar_bubble_delegate.h"
 #include "extensions/browser/extension_registry.h"
@@ -57,6 +58,7 @@
   void OnBubbleClosed(CloseAction action) override;
 
   const extensions::Extension* extension_for_testing() {
+    CHECK_IS_TEST();
     return extension_.get();
   }
   // Don't try to navigate when "learn more" is clicked.