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
cd678fb591