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

Review URL: https://codereview.chromium.org/1358763004

Cr-Commit-Position: refs/heads/master@{#350212}
4 files changed