This patch:
- moves ffmpeg binaries from `//bin/` to `//third_party/ffmpeg`
- adds [COPYING.GPLv3](https://github.com/FFmpeg/FFmpeg/blob/master/COPYING.GPLv3)
ffmpeg license
- changes npm packaging to include `//third_party/ffmpeg` only in `playwright` and `playwrihgt-chromium` a
This patch:
- adds FFMPEG binaries to the NPM packages
- adds a screencast test to make sure that screencast works. This currently relies on private screencast APIs.
NOTE: with this patch playwright package size grows from `650KB` to `4.2MB`.
