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/rtc_base/asyncudpsocket.cc b/rtc_base/asyncudpsocket.cc
index 9cd9e14..2f9011c 100644
--- a/rtc_base/asyncudpsocket.cc
+++ b/rtc_base/asyncudpsocket.cc
@@ -122,9 +122,8 @@
 
   // TODO: Make sure that we got all of the packet.
   // If we did not, then we should resize our buffer to be large enough.
-  SignalReadPacket(
-      this, buf_, static_cast<size_t>(len), remote_addr,
-      (timestamp > -1 ? PacketTime(timestamp, 0) : CreatePacketTime(0)));
+  SignalReadPacket(this, buf_, static_cast<size_t>(len), remote_addr,
+                   (timestamp > -1 ? timestamp : TimeMicros()));
 }
 
 void AsyncUDPSocket::OnWriteEvent(AsyncSocket* socket) {