Proposal: flexible chromium revisions

[Glennvd]

Often we want to test changes on new chromium versions. At the moment we would have to switch from the default downloader to specifying our own executable and downloading that etc.

Exposing this to an environment variable would make it easy to test against different chromium versions in CI environments.

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[Glennvd]

CLA is signed under Social Significance LTD

[ebidel]

Note, we do have a BrowserFetcher API that serves this purpose:
https://github.com/GoogleChrome/puppeteer/blob/master/docs/api.md#class-browserfetcher

But this one liner is a nicer than writing some code. @aslushnikov wdyt?

We'd need to also document this env var in https://github.com/GoogleChrome/puppeteer/blob/master/docs/api.md#environment-variables.

[Glennvd]

I've added the documentation as well FYI

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CLAs look good, thanks!