This makes it easier to reason about our packages.
The only difference is what each package downloads.
When the browser is not downloaded, it will fail to launch.
Each browser gets a 'download' attribute in the browser.json file.
This patch:
- removes `browserType.downloadBrowserIfNeeded()` method. The method
turned out to be ill-behaving and cannot not be used as we'd like to (see #1085)
- adds a `browserType.setExecutablePath` method to set a browser
exectuable.
With this patch, we take the following approach towards managing browser downloads:
- `playwright-core` doesn't download any browsers. In `playwright-core`, `playwright.chromium.executablePath()` returns `null` (same for firefox and webkit).
- clients of `playwright-core` (e.g. `playwright` and others) download browsers one way or another.
They can then configure `playwright` with executable paths and re-export the `playwright` object to their clients.
- `playwright`, `playwright-firefox`, `playwright-chromium` and `playwright-webkit` download
browsers. Once browsers are downloaded, their executable paths are saved to a `.downloaded-browsers.json` file. This file is read in `playwright/index.js` to configure browser executable paths and re-export the API.
- special case is `install-from-github.js` that also cleans up old browsers.
