Using microseconds in FakeNetworkPipe.

This CL changes the usages of milliseconds as a unit in FakeNetworkPipe
to microseconds. This matches the time unit of the PacketTime struct and
increases the precision of the simulation. The time resolution in
FakeNetworkPipe::Config is kept unchanged to keep the values more human
readable.

This CL prepares refactoring in upcoming CLs.

Bug: webrtc:9054
Change-Id: I103f7a0afa41381f676ea07fcc8c083532e61f1d
Reviewed-on: https://webrtc-review.googlesource.com/64140
Commit-Queue: Sebastian Jansson <srte@webrtc.org>
Reviewed-by: Niels Moller <nisse@webrtc.org>
Reviewed-by: Christoffer Rodbro <crodbro@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23018}