absl/time/duration_test.cc uses isnan/isinf from std namespace w/o qualifier, fails to build w/ gcc 5.4.0

[vsrinivas]

On Ubuntu 16.04, g++ 5.4.0, libstdc++6 5.4.0, //absl/time:time_test fails to build; duration_test.cc uses isnan and isinf from the std namespace (from cmath) without std:: qualification.

[derekmauro]

This should be fixed at HEAD.