bluetooth: Blink IDL now supports Promise<BluetoothDevice>

Blink now supports Promise<T> syntax, crbug.com/421539.
Update BluetoothDiscovery.idl to use it.

BUG=420284

Review URL: https://codereview.chromium.org/671713002

git-svn-id: svn://svn.chromium.org/blink/trunk@184122 bbb929c8-8fbe-4397-9dbb-9b2b20218538