Function build_gate 

pub fn build_gate(
    policy: DevToolsPolicy,
    main_label: &str,
    registrations: &[ContentWebview],
    devtools_compiled: bool,
) -> Result<DevToolsGate, DevToolsError>

Validate the policy and the content registrations and build the gate. devtools_compiled is cfg!(feature = "content-devtools") in the shell; it is a parameter so tests can cover both.