Update the device permissions prompt when the device list changes.

Update the device permissions prompt when the device list changes.

This patch sets up DevicePermissionsPrompt::Prompt as a
UsbService::Observer so that the UI is updated whenever a device
is connected or disconnected.

BUG=411715
Committed: https://crrev.com/6871112deb52f0469df3b9b2d8f11f2a78ee89de
Cr-Commit-Position: refs/heads/master@{#310157}

[Reilly Grant (use Gerrit), 2015-01-05 19:31:04]

reillyg@chromium.org changed reviewers:
+ yoz@chromium.org

[Reilly Grant (use Gerrit), 2015-01-05 19:31:26]

Yoyo, please take a look at this patch.

[Yoyo Zhou, 2015-01-06 00:01:19]

LGTM

[Reilly Grant (use Gerrit), 2015-01-06 21:20:08]

The CQ bit was checked by reillyg@chromium.org

[commit-bot: I haz the power, 2015-01-06 21:21:12]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/839453002/20001

[commit-bot: I haz the power, 2015-01-06 22:11:25]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-01-06 22:12:12]

Patchset 2 (id:??) landed as
https://crrev.com/6871112deb52f0469df3b9b2d8f11f2a78ee89de
Cr-Commit-Position: refs/heads/master@{#310157}