[CRD iOS] Don't attempt to connect to an offline host

Previously when the user clicks on an offline host, the client will still
attempts to connect to it, then it crashes.

This CL prevents the client from attempting to connect to an offline host.
When the offline host is selected, the client will just show a snack bar
message and do nothing else.

Review-Url: https://codereview.chromium.org/2914233003
Cr-Commit-Position: refs/heads/master@{#476455}

remoting/ios/app/remoting_view_controller.mm

@@ -203,6 +203,13 @@ static CGFloat kHostInset = 5.f;
 
 - (void)didSelectCell:(HostCollectionViewCell*)cell
            completion:(void (^)())completionBlock {
+  if (![cell.hostInfo isOnline]) {
+    MDCSnackbarMessage* message = [[MDCSnackbarMessage alloc] init];
+    message.text = @"Host is offline.";
+    [MDCSnackbarManager showMessage:message];
+    return;
+  }
+
   _client = [[RemotingClient alloc] init];
 
   [_remotingService.authentication