DHCPMessage: support DHCP option 'requested ip address'

This allows the client to request that a particular IP
address be assigned.

Bug: 25642025
TEST=compile and unittes
Change-Id: Icfb44480c79fc8810f6d3c064a6cad8e9b94184d
diff --git a/dhcp_message.h b/dhcp_message.h
index 39b3929..2275105 100644
--- a/dhcp_message.h
+++ b/dhcp_message.h
@@ -74,6 +74,7 @@
   void SetMessageType(uint8_t message_type);
   void SetParameterRequestList(
       const std::vector<uint8_t>& parameter_request_list);
+  void SetRequestedIpAddress(uint32_t requested_ip_address);
   void SetServerIdentifier(uint32_t server_identifier);
   void SetTransactionID(uint32_t transaction_id);
   void SetVendorSpecificInfo(const shill::ByteString& vendor_specific_info);
@@ -155,6 +156,8 @@
   std::string domain_name_;
   // Option 43: Vendor Specific Information.
   shill::ByteString vendor_specific_info_;
+  // Option 50: Requested IP Address.
+  uint32_t requested_ip_address_;
   // Option 51: IP address lease time in unit of seconds.
   uint32_t lease_time_;
   // Option 53: DHCP message type.