Treat negative ntp time as unset.
Treat negative ntp time as unset.

The video send pipeline uses the magic value 0 for an unset ntp time.
However, the receive pipeline uses the magic value -1 for unset
(unclear where, it seems it behaved differently a few months ago).

This makes cl https://codereview.webrtc.org/2469993003/ fail the
P2PTestConductor.ForwardVideoOnlyStream, because that cl removes code
which always clears the ntp time, and enables propagation of ntp time
from the receive pipeline to the send pipeline.

Treating ntp time <= 0 as unset is a small improvement. Ultimately, a
VideoFrame shouldn't carry an ntp time at all.

BUG=webrtc:5740,webrtc:6977

Review-Url: https://codereview.webrtc.org/2620383005
Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#16035}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 891419f8e8a1b0fa2d53c887a033693652ff7d28

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