Remove PowerManagerClient::WaitForServiceToBecomeAvailable()

Replace with PowerManagerClient::Observer::PowerManagerBecameAvailable()

This should make it easier to support for the Mojo PowerManagerClient.

Bug: 644348
Change-Id: I5c18e76eb34929a43769b6206ed16e8bc7fca4bd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1574681
Reviewed-by: Jia Meng <jiameng@chromium.org>
Reviewed-by: Dan Erat <derat@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#653645}