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.
This establishes a single naming for all our blobs with browser
builds that we upload to CDN: `<browser-name>-<os-version>`
- `<browser-name>` is either `firefox` or `webkit`.
- `os-version` is the OS that was used to produce the build.
References #2745
Currently, it might happen that two different patches clash for the
same build number for the browsers. In this case, authors might
not even know that they need to rebaseline.
This patch starts adding a second line to `BUILD_NUMBER` files - the
signature and date of the `BUILD_NUMBER` change. These are guaranteed
to clash, so it should not be possible to land patches without
re-baselining them.
