We had (and still have) a ton of pull requests to support
PUPPETEER_EXECUTABLE_PATH and PUPPETEER_CHROMIUM_REVISION in puppeteer launcher.
We were hesitant before since env variables are not scoped
and thus don't make a good interface for a library. Now, since we
determined `puppeteer-core` as a library and `puppeteer` as our end-user
product, it's safe to satisfy our user needs.
This patch:
- teaches PUPPETEER_EXECUTABLE_PATH and PUPPETEER_CHROMIUM_REVISION
env variables to control how Puppeteer launches browser
- makes sure these variables play no role in `puppeteer-core` package.
28d92116b7