Do not try to make Chrome default when registering it if it can't be made default unattended.

chrome/installer/setup/install.h

 // Copyright (c) 2012 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.
 //
 // This file contains the specification of setup main functions.
 
 #ifndef CHROME_INSTALLER_SETUP_INSTALL_H_
 #define CHROME_INSTALLER_SETUP_INSTALL_H_
 
 #include <vector>
@@ skipping 62 matching lines @@
 // If creating the Start menu shortcut is successful, it is also pinned to the
 // taskbar.
 void CreateOrUpdateShortcuts(
     const base::FilePath& target,
     const Product& product,
     const MasterPreferences& prefs,
     InstallShortcutLevel install_level,
     InstallShortcutOperation install_operation);
 
 // Registers Chrome on this machine.
-// If |make_chrome_default|, also attempts to make Chrome default (potentially
-// popping a UAC if the user is not an admin and HKLM registrations are required
-// to register Chrome's capabilities on this version of Windows (i.e.
-// pre-Win8)).
+// If |make_chrome_default|, also attempts to make Chrome default where doing so
+// requires no more user interaction than a UAC prompt. In practice, this means
+// on versions of Windows prior to Windows 8.
 void RegisterChromeOnMachine(const InstallerState& installer_state,
                              const Product& product,
                              bool make_chrome_default);
 
 // This function installs or updates a new version of Chrome. It returns
 // install status (failed, new_install, updated etc).
 //
 // setup_path: Path to the executable (setup.exe) as it will be copied
 //           to Chrome install folder after install is complete
 // archive_path: Path to the archive (chrome.7z) as it will be copied
@@ skipping 37 matching lines @@
                                  bool force);
 
 // Launches app_host.exe to install content from web store (non-blocking).
 // Returns true on successful execution (although successful installation
 // is not guaranteed).
 bool InstallFromWebstore(const std::string& app_code);
 
 }  // namespace installer
 
 #endif  // CHROME_INSTALLER_SETUP_INSTALL_H_