Fix NetworkEmulationManagerTest.ThroughputStats flakiness.
Fix NetworkEmulationManagerTest.ThroughputStats flakiness.

Account for time measurement variability.

Bug: webrtc:10553
Change-Id: I7a82a15d5a7c2fb3e5cb80bfdf140433a3b93349
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/151780
Commit-Queue: Yves Gerey <0554266f02d7ad5bb902ac47b1c8525c05c3857c@google.com>
Reviewed-by: Artem Titov <1e27363ccc366a7ff0fdde48204d68d5ebfeccc5@webrtc.org>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#29101}

TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc