Make getBluetoothManualChooserEvents() asynchronous.

This fixes the Windows tests, where synchronous, unpumped messages to the UI thread are forbidden: https://codereview.chromium.org/1304353004/

BUG=500989

Review URL: https://codereview.chromium.org/1351393002

Cr-Commit-Position: refs/heads/master@{#350167}
10 files changed