Add functions to let tests read and control the Bluetooth chooser state.

These functions are specified at https://webbluetoothcg.github.io/web-bluetooth/tests/.

See tests using this in https://codereview.chromium.org/1304353004/.

BUG=500989

Review URL: https://codereview.chromium.org/1325953002

Cr-Commit-Position: refs/heads/master@{#348202}
16 files changed