Add chrome.bluetooth and chrome.bluetoothLowEnergy for iOS

I also create a peripheral testing app that simply simulate BLE heart rate sensor, and I will add more to simulate other things for more tests.
https://github.com/oahziur/ble-tests-helper

I removed `chrome.bluetooth.startDiscovery` and `chrome.bluetooth.stopDiscovery` for iOS because I found that low energy devices can be found without calling them (which conflict with: https://developer.chrome.com/apps/app_bluetooth#le-discovery) on ChromeOS.
Purpose of this method isn't obvious from its name. Can you add a comment saying what it does?