Don't create a pairing registry on platforms that don't provide a delegate.

This was broken by https://codereview.chromium.org/834673007. Constructing
a PairingRegistry with a null delegate causes pairing to be offered, but if
selected, the host process will crash.

BUG=455501

Review URL: https://codereview.chromium.org/903963002

Cr-Commit-Position: refs/heads/master@{#314969}

remoting/host/remoting_me2me_host.cc

@@ -629,6 +629,7 @@ void HostProcess::CreateAuthenticatorFactory() {
         scoped_ptr<PairingRegistry::Delegate> delegate =
             CreatePairingRegistryDelegate();
 
+        if (delegate)
           pairing_registry_ = new PairingRegistry(context_->file_task_runner(),
                                                   delegate.Pass());
       }