Revert of Delete class MockCongestionController. (patchset #4 id:60001 of https://codereview.webrtc.org/2762023004/ )

Reason for revert:
This is breaking downstream build.

Original issue's description:
> Delete class MockCongestionController.
>
> It became unused in cl https://codereview.webrtc.org/2516983004.
>
> BUG=None
>
> Review-Url: https://codereview.webrtc.org/2762023004
> Cr-Commit-Position: refs/heads/master@{#17325}
> Committed: https://chromium.googlesource.com/external/webrtc/+/d19bcb7116d527b5d0b7426468490e72b03358ca

TBR=stefan@webrtc.org,nisse@webrtc.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=None

Review-Url: https://codereview.webrtc.org/2762133003
Cr-Original-Commit-Position: refs/heads/master@{#17330}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: e27f1e764ef45f6489cf192de843ca912a05a101
4 files changed