GitLab

When BluetoothChooser object is destroyed, its associated Bluetooth
EventHandler becomes invalid too and should not be used any more.
This CL resets the Bluetooth EventHandler when it becomes invalid
so that it can't be used any more.

BUG=629298

Review-Url: https://codereview.chromium.org/2203103003
Cr-Commit-Position: refs/heads/master@{#410690}