Initial polyfill on top of the Chrome Apps API

https://github.com/WebBluetoothCG/chrome-app-polyfill now has a rough
polyfill of the current API (without most of the 'needs wording' issues) on
top of the chrome.bluetooth API. I won't be able to give it much love over
the next month, but there should be enough folks around who can accept pull
requests if anyone has time to complete it, add tests, etc.

I'm hoping to add a Cordova polyfill too, but that's probably going to have
lower priority than getting the Chromium implementation working behind a
flag.

Jeffrey