On Chrome OS, updates about the system power status are
received via D-Bus messages on the UI thread. Being in
base/, PowerMonitor can't depend on the code in chromeos/
that receives those messages, so make the Chrome OS code
instead notify PowerMonitor when it receives updates.

BUG=326534

Review URL: https://codereview.chromium.org/622693004

Cr-Commit-Position: refs/heads/master@{#298367}