Add implicit conversion between rtc:PacketTime and int64_t.

This is a preparation for deleting rtc::PacketTime. Next step, after
downstream code has been updated to not access the |timestamp| member,
is to make rtc::PacketTime an alias for int64_t.

Also delete the unused member rtc::PacketTime::not_before.

Bug: webrtc:9584
Change-Id: Iba9d2d55047d69565ad62b1beb525591fd432ae2
Reviewed-on: https://webrtc-review.googlesource.com/c/108860
Commit-Queue: Niels Moller <nisse@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25468}
diff --git a/p2p/base/asyncstuntcpsocket.cc b/p2p/base/asyncstuntcpsocket.cc
index 8bb796f..d0185c3 100644
--- a/p2p/base/asyncstuntcpsocket.cc
+++ b/p2p/base/asyncstuntcpsocket.cc
@@ -112,7 +112,7 @@
     }
 
     SignalReadPacket(this, data, expected_pkt_len, remote_addr,
-                     rtc::CreatePacketTime(0));
+                     rtc::TimeMicros());
 
     *len -= actual_length;
     if (*len > 0) {