Add smoke test for UsbService::GetDevices.

Add smoke test for UsbService::GetDevices.

This test initializes the USB service and calls GetDevices() to exercise
the native USB device enumeration path on all platforms. Since we cannot
guarantee that a particular piece of hardware is connected the result
of this call is not checked. Tested enabled by --enable-gadget-tests
provide deeper converage when matching hardware is available.

BUG=None

Committed: https://crrev.com/1812419e01d1ff037574323597e4508a6660e9a6
Cr-Commit-Position: refs/heads/master@{#397850}

[Reilly Grant (use Gerrit), 2016-06-03 22:38:01]

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

[Reilly Grant (use Gerrit), 2016-06-03 22:38:02]

Please take a look.

[juncai, 2016-06-03 23:27:14]

LGTM.

[Reilly Grant (use Gerrit), 2016-06-03 23:41:03]

The CQ bit was checked by reillyg@chromium.org

[commit-bot: I haz the power, 2016-06-03 23:41:36]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/2032303003/1

[commit-bot: I haz the power, 2016-06-04 00:03:57]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2016-06-04 00:07:09]

Description was changed from

==========
Add smoke test for UsbService::GetDevices.

This test initializes the USB service and calls GetDevices() to exercise
the native USB device enumeration path on all platforms. Since we cannot
guarantee that a particular piece of hardware is connected the result
of this call is not checked. Tested enabled by --enable-gadget-tests
provide deeper converage when matching hardware is available.

BUG=None
==========

to

==========
Add smoke test for UsbService::GetDevices.

This test initializes the USB service and calls GetDevices() to exercise
the native USB device enumeration path on all platforms. Since we cannot
guarantee that a particular piece of hardware is connected the result
of this call is not checked. Tested enabled by --enable-gadget-tests
provide deeper converage when matching hardware is available.

BUG=None

Committed: https://crrev.com/1812419e01d1ff037574323597e4508a6660e9a6
Cr-Commit-Position: refs/heads/master@{#397850}
==========

[commit-bot: I haz the power, 2016-06-04 00:07:10]

Patchset 1 (id:??) landed as
https://crrev.com/1812419e01d1ff037574323597e4508a6660e9a6
Cr-Commit-Position: refs/heads/master@{#397850}