Skip attempting to bind a DevicePtr for a removed device.

This patch skips calling DeviceManager::GetDevice for a removed device
as this is guaranteed to fail. Instead a null DevicePtr is simply passed
to WebUSBDeviceImpl which has been updated to handle this.

BUG=492204

Review URL: https://codereview.chromium.org/1755683004

Cr-Commit-Position: refs/heads/master@{#378856}
3 files changed