[Reland][Extensions] Enable sideloading prompt on mac on canary

chrome/browser/extensions/external_install_manager.h

Index: chrome/browser/extensions/external_install_manager.h
diff --git a/chrome/browser/extensions/external_install_manager.h b/chrome/browser/extensions/external_install_manager.h
index 99c60cb4d11730a3e09e6167c099b2fec60fd7e5..0f0b73ddff278ffea887ddb534a9d0d77f53dd3a 100644
--- a/chrome/browser/extensions/external_install_manager.h
+++ b/chrome/browser/extensions/external_install_manager.h
@@ -34,6 +34,9 @@ class ExternalInstallManager : public ExtensionRegistryObserver,
                          bool is_first_run);
   ~ExternalInstallManager() override;
 
+  // Returns true if prompting for external extensions is enabled.
+  static bool IsPromptingEnabled();
+
   // Removes the error associated with a given extension.
   void RemoveExternalInstallError(const std::string& extension_id);