Add a kill switch for all of Web Bluetooth.

This is temporary until we get Web Bluetooth behind
PermissionContextBase, but that may block on getting some UI done, so I
don't want to bet on it happening by M51.

BUG=598059

Review URL: https://codereview.chromium.org/1841763002

Cr-Commit-Position: refs/heads/master@{#384162}
13 files changed