This patch moves FFMPEG building to buildbots:
- `ffmpeg-mac.zip` is built on Mac 10.14 machine
- `ffmpeg-win32.zip` and `ffmpeg-win64.zip` are cross-compiled on
Ubuntu 20.04 machine
All builds across the platforms share the same config:
- the same versions of `ffmpeg` and `libvpx`
- the same build configuration for both `ffmpeg` and `libvpx`
The config could be found in the `//browser_patches/ffmpeg/CONFIG.sh`.
The builds will be then copied manually and committed to the git
repository.
