Control the availability of developer mode on extensions page

Control if users can turn on Developer Mode on chrome://extensions. If the policy is not set, users can turn on developer mode on extension page unless DeveloperToolsAvailability policy is set to DeveloperToolsDisallowed (2). If the policy is set to Allow (0), users can turn on developer mode on extensions page. If the policy is set to Disallow (1), users can not turn on developer mode on extensions page. If this policy is set, DeveloperToolsAvailability can no longer control extensions developer mode. Note: This policy only applies to platforms that support extensions.