Fix NetworkEmulationManagerTest.ThroughputStats flakiness (again).
Fix NetworkEmulationManagerTest.ThroughputStats flakiness (again).

This CL increases tolerance for time measurement variability,
from 1% to 5%. This accounts for heavily loaded bots in CI.

Bug: webrtc:10553
Change-Id: I014f856f9e358f2871a2df8bc39ab0995fdc971f
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/159284
Reviewed-by: Artem Titov <1e27363ccc366a7ff0fdde48204d68d5ebfeccc5@webrtc.org>
Commit-Queue: Yves Gerey <0554266f02d7ad5bb902ac47b1c8525c05c3857c@google.com>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#29765}

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