Create a FakeTetherService, which stubs out TetherService for development.

This change incidentally also restarts Tether after a Chrome crash, which was overlooked previously.

Usage: pass --tether-stub=N as a runtime argument to enable FakeTetherService and create N fake Tether networks. Be sure to also pass --enable-tether, and enable Bluetooth.

BUG=672263

Review-Url: https://codereview.chromium.org/2900903002
Cr-Commit-Position: refs/heads/master@{#474555}
12 files changed