Make base/ui/idle a separate component

idle_chromeos.cc depends on SessionManagerClient which is being removed
from Ash. This code is not currently used in ash, so this CL extracts
that code to a separate component explicitly disallowed in Ash.

Bug: 644350
Change-Id: Ia32515f1d12c25b7a9c1459a344e2321facb3202
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1524646
Commit-Queue: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Dan Erat <derat@chromium.org>
Cr-Commit-Position: refs/heads/master@{#641221}
9 files changed