shill: L2TPIPsecDriver: Specify smaller MTU

L2TP packets have additional header information that count
against the available link MTU.  Although this MTU information
isn't passed down as a parameter from the VPN client, we should
account for this by using a failsafe value.

BUG=chromium:450759
TEST=Unit tests, Upcoming test network_DhcpMTU (CL:243315)

Change-Id: Ica22f148679cea908a7761c6b6ba7b619355a22f
Reviewed-on: https://chromium-review.googlesource.com/243371
Reviewed-by: Prabhu Kaliamoorthi <kaliamoorthi@chromium.org>
Reviewed-by: Paul Stewart <pstew@chromium.org>
Tested-by: Paul Stewart <pstew@chromium.org>
Commit-Queue: Paul Stewart <pstew@chromium.org>
diff --git a/ppp_device.h b/ppp_device.h
index fd88855..d731d00 100644
--- a/ppp_device.h
+++ b/ppp_device.h
@@ -44,6 +44,13 @@
       const std::map<std::string, std::string> &configuration,
       bool blackhole_ipv6);
 
+  // Same as UpdateIPConfigFromPPP except overriding the default MTU
+  // in the IPConfig.
+  virtual void UpdateIPConfigFromPPPWithMTU(
+      const std::map<std::string, std::string> &configuration,
+      bool blackhole_ipv6,
+      int32_t mtu);
+
   // Get the network device name (e.g. "ppp0") from the dictionary of
   // configuration strings received from our PPP plugin.
   static std::string GetInterfaceName(