Reland of Delete class MockCongestionController. (patchset #1 id:1 of https://codereview.webrtc.org/2762133003/ )

Reason for revert:
Problem was the rename of the include file. Intend to keep the old name for now, and then reland.

Original issue's description:
> 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-Commit-Position: refs/heads/master@{#17330}
> Committed: https://chromium.googlesource.com/external/webrtc/+/e27f1e764ef45f6489cf192de843ca912a05a101

TBR=stefan@webrtc.org,skvlad@webrtc.org
BUG=None

Review-Url: https://codereview.webrtc.org/2766133002
Cr-Original-Commit-Position: refs/heads/master@{#17338}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 40854eab23e27bdb7aebd98fb1ecd6198fec97de
4 files changed