Implement getConfiguration in WebUSBDeviceImpl.

Implement getConfiguration in WebUSBDeviceImpl.

This USBDevice method allows a site to determine the current
configuration state of the device. A follow-up to this change will make
WebUSBDevice::getConfiguration a pure virtual method.

Blink change: https://codereview.chromium.org/1352283005/

BUG=492204
Committed: https://crrev.com/8b606624ecade50e8b5be2c9d423c976ac23ce7a
Cr-Commit-Position: refs/heads/master@{#350212}

[Reilly Grant (use Gerrit), 2015-09-22 00:21:20]

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

[Reilly Grant (use Gerrit), 2015-09-22 00:21:21]

PTAL

[Ken Rockot(use gerrit already), 2015-09-22 17:47:36]

lgtm

[Reilly Grant (use Gerrit), 2015-09-22 17:57:59]

The CQ bit was checked by reillyg@chromium.org

[commit-bot: I haz the power, 2015-09-22 17:58:34]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1358763004/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1358763004/1

[commit-bot: I haz the power, 2015-09-22 19:30:33]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-09-22 19:31:19]

Patchset 1 (id:??) landed as
https://crrev.com/8b606624ecade50e8b5be2c9d423c976ac23ce7a
Cr-Commit-Position: refs/heads/master@{#350212}