chromeos: Simplify ShutdownPolicyHandler

chromeos: Simplify ShutdownPolicyHandler

This is preparation for moving the actual shutdown code into ash.

ShutdownPolicyHandler observes the kRebootOnShutdown policy setting. It
notifies observers in OOBE and the ash system tray when the value changes.
It also has a callback-based interface to get the current value.

Simplify the implementation by eliminating the callback-based interface
and only using the delegate.

BUG=644348, 647412
TEST=chrome unit_tests, instrumenting the code to manually change the policy
setting and observing the tooltips on the shutdown button

[James Cook, 2016-11-10 23:33:54]

jamescook@chromium.org changed reviewers:
+ derat@chromium.org

[James Cook, 2016-11-10 23:33:55]

Dan, please take a look.

https://codereview.chromium.org/2492063002/diff/20001/ash/common/system/tiles...
File ash/common/system/tiles/tiles_default_view.cc (right):

https://codereview.chromium.org/2492063002/diff/20001/ash/common/system/tiles...
ash/common/system/tiles/tiles_default_view.cc:116: #endif  // !defined(OS_WIN)
FYI - The above code is forked/copied because it's the new material design
system tray menu. Material design is on by default in M56, so the other file
will be deleted soon. I didn't fork it. :-)

[James Cook, 2016-11-10 23:34:07]

The CQ bit was checked by jamescook@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2016-11-10 23:34:55]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-11-11 00:07:58]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2016-11-11 00:07:59]

Dry run: This issue passed the CQ dry run.

[Daniel Erat, 2016-11-11 00:13:00]

lgtm with a naming question

https://codereview.chromium.org/2492063002/diff/20001/chrome/browser/chromeos...
File chrome/browser/chromeos/settings/shutdown_policy_handler.h (right):

https://codereview.chromium.org/2492063002/diff/20001/chrome/browser/chromeos...
chrome/browser/chromeos/settings/shutdown_policy_handler.h:35: void
CheckIfRebootOnShutdown();
just thinking aloud here, but would a name like
NotifyDelegateIfShutdownPolicyChanged be clearer? "check" feels a bit ambiguous.
i sometimes name methods "check" when they CHECK() something. :-P

[James Cook, 2016-11-11 16:59:43]

Patchset #4 (id:60001) has been deleted

[James Cook, 2016-11-11 17:34:04]

Thanks for the quick review. I'm going to hold off on landing this for now. It
looks like my follow-up patch to move shutdown code into //ash is going to touch
almost all of these lines again, so it's probably not worth the churn to land
two patches. I'll see how it goes.