feat(install): introduce PUPPETEER_CHROMIUM_REVISION (puppeteer#2246)

This patch introduces PUPPETEER_CHROMIUM_REVISION environment variable
to specify chromium revision to be downloaded during install step.

docs/api.md

@@ -270,6 +270,7 @@ Puppeteer looks for certain [environment variables](https://en.wikipedia.org/wik
 - `HTTP_PROXY`, `HTTPS_PROXY`, `NO_PROXY` - defines HTTP proxy settings that are used to download and run Chromium.
 - `PUPPETEER_SKIP_CHROMIUM_DOWNLOAD` - do not download bundled Chromium during installation step.
 - `PUPPETEER_DOWNLOAD_HOST` - overwrite host part of URL that is used to download Chromium
+- `PUPPETEER_CHROMIUM_REVISION` - specify a certain version of chrome you'd like puppeteer to use during the installation step.
 
 ### class: Puppeteer
 

install.js

@@ -30,7 +30,7 @@ const downloadHost = process.env.PUPPETEER_DOWNLOAD_HOST || process.env.npm_conf
 const puppeteer = require('./index');
 const browserFetcher = puppeteer.createBrowserFetcher({ host: downloadHost });
 
-const revision = require('./package.json').puppeteer.chromium_revision;
+const revision = process.env.PUPPETEER_CHROMIUM_REVISION || require('./package.json').puppeteer.chromium_revision;
 const revisionInfo = browserFetcher.revisionInfo(revision);
 
 // Do nothing if the revision is already downloaded.