Implement WebUSBClient::setObserver in WebUSBClientImpl.

This patch updates WebUSBClientImpl to use the new Mojo
DeviceManager::GetDeviceChanges interface to send USB device connection
events to the observer provided by Blink.

BUG=492204

Review URL: https://codereview.chromium.org/1331383005

Cr-Commit-Position: refs/heads/master@{#349137}
2 files changed