reducing the usage of RTCStats-helper.js. MTI stats are tested in
  RTCPeerConnection-mandatory-getStats.https.html

BUG=chromium:1395574

Change-Id: I0402e672341408b80b7bb9163d0032f7c6cd8d80
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5245568


Reviewed-by: Henrik Boström <hbos@chromium.org>
Commit-Queue: Philipp Hancke <phancke@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1253798}