[Extensions] Enable sideloading prompt on mac on canary

chrome/browser/extensions/external_install_manager.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CHROME_BROWSER_EXTENSIONS_EXTERNAL_INSTALL_MANAGER_H_
 #define CHROME_BROWSER_EXTENSIONS_EXTERNAL_INSTALL_MANAGER_H_
 
 #include <map>
 #include <memory>
 
@@ skipping 16 matching lines @@
 class ExtensionPrefs;
 class ExternalInstallError;
 
 class ExternalInstallManager : public ExtensionRegistryObserver,
                                public content::NotificationObserver {
  public:
   ExternalInstallManager(content::BrowserContext* browser_context,
                          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);
 
   // Checks if there are any new external extensions to notify the user about.
   void UpdateExternalExtensionAlert();
 
   // Given a (presumably just-installed) extension id, mark that extension as
   // acknowledged.
   void AcknowledgeExternalExtension(const std::string& extension_id);
 
@@ skipping 66 matching lines @@
 
   ScopedObserver<ExtensionRegistry, ExtensionRegistryObserver>
       extension_registry_observer_;
 
   DISALLOW_COPY_AND_ASSIGN(ExternalInstallManager);
 };
 
 }  // namespace extensions
 
 #endif  // CHROME_BROWSER_EXTENSIONS_EXTERNAL_INSTALL_MANAGER_H_