athena: Use ActivityList where appropriate, and fix AppActivity tests.

athena/content/app_activity.h

Index: athena/content/app_activity.h
diff --git a/athena/content/app_activity.h b/athena/content/app_activity.h
index 37ffd208b5c5a06b71019851682e67dfda6d8dfe..771a63405c34eee9344c5cda1ff1be7d14a66111 100644
--- a/athena/content/app_activity.h
+++ b/athena/content/app_activity.h
@@ -57,15 +57,19 @@ class AppActivity : public Activity,
 
   ~AppActivity() override;
 
+  // Register this activity with its application.
+  void RegisterActivity();
+
+  AppActivityRegistry* app_activity_registry() {
+    return app_activity_registry_;
+  }
+
  private:
  // content::WebContentsObserver:
   void TitleWasSet(content::NavigationEntry* entry, bool explicit_set) override;
   void DidUpdateFaviconURL(
       const std::vector<content::FaviconURL>& candidates) override;
 
-  // Register this activity with its application.
-  void RegisterActivity();
-
   // Hiding the contet proxy and showing the real content instead.
   void HideContentProxy();