Bump up the timeout on port acquisition and forwaring to 60 seconds. (#6468)

It takes a little bit longer that 10 seconds to install, launch and
have the observatory be available on an iPod touch. Depending on the
size of the application, it could be a lot longer to transer the
bundle over the wire. The 60 seconds is arbitrary.

packages/flutter_tools/lib/src/ios/devices.dart

@@ -295,7 +295,7 @@ class IOSDevice extends Device {
   }
 
   Future<int> _acquireAndForwardPort(String serviceName, int localPort) async {
-    Duration stepTimeout = const Duration(seconds: 10);
+    Duration stepTimeout = const Duration(seconds: 60);
 
     Future<int> remote = new ProtocolDiscovery(logReader, serviceName).nextPort();