chore(juggler): Roll Firefox to 120450a2 (#3842)

This patch:
- rebaselines C++ patchset atop of [Global Firefox Reformat](https://bugzilla.mozilla.org/show_bug.cgi?id=1511181)
- rolls firefox to [120450a2](120450a2c5)
- splits out preference installation step from Puppeteer-Firefox's
  `install.js` into a separate
  `puppeteer-firefox/misc/install-preferences.js`. This script is
  re-used to install preferences when run with a custom executable path.
- fixes issue with ScrollbarManager that was re-injecting the same
  stylesheet multiple times

experimental/puppeteer-firefox/misc/install-preferences.js

@@ -0,0 +1,57 @@
+const os = require('os');
+const fs = require('fs');
+const path = require('path');
+
+// Install browser preferences after downloading and unpacking
+// firefox instances.
+// Based on:   https://developer.mozilla.org/en-US/docs/Mozilla/Firefox/Enterprise_deployment_before_60#Configuration
+async function installFirefoxPreferences(executablePath) {
+  const firefoxFolder = path.dirname(executablePath);
+  const {helper} = require('../lib/firefox/helper');
+  const mkdirAsync = helper.promisify(fs.mkdir.bind(fs));
+
+  let prefPath = '';
+  let configPath = '';
+  if (os.platform() === 'darwin') {
+    prefPath = path.join(firefoxFolder, '..', 'Resources', 'defaults', 'pref');
+    configPath = path.join(firefoxFolder, '..', 'Resources');
+  } else if (os.platform() === 'linux') {
+    await mkdirAsync(path.join(firefoxFolder, 'browser', 'defaults'));
+    await mkdirAsync(path.join(firefoxFolder, 'browser', 'defaults', 'preferences'));
+    prefPath = path.join(firefoxFolder, 'browser', 'defaults', 'preferences');
+    configPath = firefoxFolder;
+  } else if (os.platform() === 'win32') {
+    prefPath = path.join(firefoxFolder, 'defaults', 'pref');
+    configPath = firefoxFolder;
+  } else {
+    throw new Error('Unsupported platform: ' + os.platform());
+  }
+
+  await Promise.all([
+    copyFile({
+      from: path.join(__dirname, '00-puppeteer-prefs.js'),
+      to: path.join(prefPath, '00-puppeteer-prefs.js'),
+    }),
+    copyFile({
+      from: path.join(__dirname, 'puppeteer.cfg'),
+      to: path.join(configPath, 'puppeteer.cfg'),
+    }),
+  ]);
+}
+
+function copyFile({from, to}) {
+  var rd = fs.createReadStream(from);
+  var wr = fs.createWriteStream(to);
+  return new Promise(function(resolve, reject) {
+    rd.on('error', reject);
+    wr.on('error', reject);
+    wr.on('finish', resolve);
+    rd.pipe(wr);
+  }).catch(function(error) {
+    rd.destroy();
+    wr.end();
+    throw error;
+  });
+}
+
+module.exports = installFirefoxPreferences;

experimental/puppeteer-firefox/misc/puppeteer.cfg

@@ -137,6 +137,9 @@ pref(
   "http://%(server)s/dummy/discoveryURL",
 );
 
+pref("extensions.screenshots.disabled", true);
+pref("extensions.screenshots.upload-disabled", true);
+
 // Allow the application to have focus even it runs in the background
 pref("focusmanager.testmode", true);