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.
