Description: 
Customer wants to restrict the Omnibox provider to be only one. Basically an extension of the DefaultSearchProviderEnabled policy, so that it doesn't only configure the default provider but also restrict the usage of additional providers.

Use case: 
Currently the DefaultSearchProviderEnabled policy (https://www.chromium.org/administrators/policy-list-3#DefaultSearchProviderEnabled)
allows the option to set a default omnibox provider, however the users can add as many as they can and use them as well by simply typing the omnibox name and pressing "space".

Motivation: 
Restricting the Omnibox provider is a step to make sure users are using Google and the respective SafeSearch filters.

Existing workarounds: 
blocking chrome://settings after configuring the the policy (will not disable already added providers by the user)