shill: Add IPv6 connection support.

Added support to shill for it to be able to complete portal detection
using IPv6 connection. The IPv6 support is added to AsyncConnection,
DNSClient, and HTTPRequest.

BUG=chromium:394010
TEST=unit tests, manual test with future CL that initiate IPv6 portal
detection.

Change-Id: I20b768211f3ecfcd53406805038c0490a488ee78
Reviewed-on: https://chromium-review.googlesource.com/213273
Reviewed-by: Peter Qiu <zqiu@chromium.org>
Tested-by: Peter Qiu <zqiu@chromium.org>
Commit-Queue: Peter Qiu <zqiu@chromium.org>
diff --git a/async_connection.h b/async_connection.h
index 216d8a2..bb5ce45 100644
--- a/async_connection.h
+++ b/async_connection.h
@@ -62,6 +62,9 @@
 
   void OnConnectCompletion(int fd);
 
+  // Initiate a socket connection to given IP address and port (in host order).
+  int ConnectTo(const IPAddress &address, int port);
+
   std::string interface_name_;
   EventDispatcher *dispatcher_;
   Sockets *sockets_;