shill: add function to check time till DHCP lease renewal

Add public function IPConfig::TimeToLeaseExpiry to check the time
left before the current DHCP lease is due to be renewed.

BUG=chromium:426657
TEST='P2_TEST_FILTER="shill::*" FEATURES="test" USE="clang asan"
emerge-samus shill' succeeds.

Change-Id: I80a912e86c677a98b48a22494e7bbc36a936cf3b
Reviewed-on: https://chromium-review.googlesource.com/225278
Reviewed-by: Samuel Tan <samueltan@chromium.org>
Commit-Queue: Samuel Tan <samueltan@chromium.org>
Tested-by: Samuel Tan <samueltan@chromium.org>
diff --git a/mock_ipconfig.h b/mock_ipconfig.h
index d35bbcc..0d97b57 100644
--- a/mock_ipconfig.h
+++ b/mock_ipconfig.h
@@ -29,6 +29,8 @@
   MOCK_METHOD0(EmitChanges, void(void));
   MOCK_METHOD1(UpdateDNSServers,
                void(const std::vector<std::string> &dns_servers));
+  MOCK_METHOD1(UpdateLeaseExpirationTime, void(uint32_t new_lease_duration));
+  MOCK_METHOD0(ResetLeaseExpirationTime, void(void));
 
  private:
   const Properties &real_properties() {