Add explicit conversions to/from uint64_t
uint64_t is natural type for NtpTime, including wrap on overflow.
BUG=None
Review-Url: https://codereview.webrtc.org/2695683002
Cr-Commit-Position: refs/heads/master@{#16624}
Seconds and fractions parts of the ntp time presented with two values, but used as one.
This helper structure can make that use more clear.
(initially introduced into rtp_rtcp as https://codereview.webrtc.org/1435833003)
BUG=webrtc:5260
Review URL: https://codereview.webrtc.org/1482593002
Cr-Commit-Position: refs/heads/master@{#10814}