GitLab

Add an observer list to HidService that is notified when a device is
added or removed from the enumeration. This currently works as expected
on Linux and OS X while on Windows devices are only enumerated on request
and so these are not as useful.

BUG=376719

Review URL: https://codereview.chromium.org/779353003

Cr-Commit-Position: refs/heads/master@{#307105}