GitLab

simulatePreconnectedPeripherals allows clients to simulate peripherals
that already connected to the system but that the UA doesn't know about.

Adds a new helper method to create system-connected devices and uses
the method in a couple of tests.

BUG=569709

Review-Url: https://codereview.chromium.org/2858803003
Cr-Commit-Position: refs/heads/master@{#471190}