This is a reland of 6a337f19713e049cd42969b0c909c59aff4146cd

This previous change caused tests to fail because of a Mojo validation
error, which is now taken care of by setting the scramble_message_ids flag in
the BUILD.gn file to false.

Original change's description:
> bluetooth: FakeBluetoothChooser impl.
>
> Implements the interfaces that FakeBluetoothChooser overrides. The
> mojom::FakeBluetoothChooser overrides are implemented by running
> |event_handler_| with the appropriate arguments. The BluetoothChooser
> overrides do nothing at the moment, since the base class is only meant
> to update the chooser user interface.
>
> This change also begins to partially convert a Bluetooth chooser test to test
> the interface and deletes the temporary tests that were created to test this
> API.
>
> This change reverts commit 42624492cd5e80855f1dd3a5a8146b258def2c2a, which
> reverted the original change. The original change caused the
> new-scan-device-added.html test to fail because of a Mojo validation error
> resulting from not updating the
> third_party/.../external/wpt/resources/chromium/fake_bluetooth_chooser.mojom.js
> file needed by the test.
>
> Bug: 719827, 719826, 824704, 826787
> Change-Id: I5e5cce7e809b4d71216c2d074a417ea822d5e527
> Reviewed-on: https://chromium-review.googlesource.com/985195
> Commit-Queue: Ovidio Henriquez <odejesush@chromium.org>
> Reviewed-by: Giovanni Ortuño Urquidi <ortuno@chromium.org>
> Reviewed-by: Daniel Cheng <dcheng@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#546729}

Bug: 719827, 719826, 824704, 826787
Change-Id: If63a681becbb2ac0f646a11ea20b1db18d42395d