iOS: Add improved BWE field trial.

BUG=webrtc:6664
NOTRY=True

Review-Url: https://codereview.webrtc.org/2482593002
Cr-Original-Commit-Position: refs/heads/master@{#14958}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: d83bf06167da78fc164b6722b5ad9258335810c8
3 files changed