issue #2702 (static_assert failure) (#2708)
issue #2702 (static_assert failure) (#2708)

* - issue #2702
- static_assert failure on underlying type check of std::chrono::nanoseconds
- The only requirement of std::chrono::nanoseconds is that it is an integral type of at least 64bits. Different implementations may have different underlying types. Therefore it is better to check whether its size is greater than or equal to sizeof(int64_t).

-fix cpplint issue

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