Disable rolling of ffmpeg.
Disable rolling of ffmpeg.

After last update in a chromium repo ffmpeg support for MSVC was broken.
So for now we will freeze rolling of ffmpeg and continue it after
we'll restore of MSVC or we'll find a way around to keep MSVC support
in the WebRTC.

Change-Id: Ie7de7e6d367946f3ad77a81d6121dd704a56ca24
Bug: webrtc:9213
Reviewed-on: https://webrtc-review.googlesource.com/78402
Reviewed-by: Patrik Höglund <b486136ee68458b09c92c86c39f451c87d6ef6a1@webrtc.org>
Commit-Queue: Artem Titov <1e27363ccc366a7ff0fdde48204d68d5ebfeccc5@webrtc.org>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#23379}
ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc