chromeos: Turn off displays on suspend

ui/display/chromeos/display_configurator.h

Index: ui/display/chromeos/display_configurator.h
diff --git a/ui/display/chromeos/display_configurator.h b/ui/display/chromeos/display_configurator.h
index 7a2449f3e56de61715d54f490333e072d18fe8d6..07e391dd62345e0f41ba43820f4656327b5f8c22 100644
--- a/ui/display/chromeos/display_configurator.h
+++ b/ui/display/chromeos/display_configurator.h
@@ -396,6 +396,9 @@ class DISPLAY_EXPORT DisplayConfigurator : public NativeDisplayObserver {
   // Stores the requested power state.
   chromeos::DisplayPowerState requested_power_state_;
 
+  // The display power state before suspend.
+  chromeos::DisplayPowerState saved_power_state_;

[Daniel Erat, 2016/04/08 20:26:51]

here's a possibly-naive question: why is this needed? it looks like
requested_power_state_ gets copied to saved_power_state_ just before the system
suspends, and then both of them get checked on resume. is this present to handle
the case where powerd sends another request during dark resume? if so, could you
document that here?

i *think* that what's going on is that SetDisplayPower() (which is usually
called by powerd) updates requested_power_state_, and you want to save the
actually-requested-by-powerd state before you call SetDisplayPower() manually so
you can restore the state later. if that's correct, please just create a new
SetDisplayPowerInternal() method. powerd can call SetDisplayPower(), which will
update the requested state and call SetDisplayPowerInternal(), while your change
can just call SetDisplayPowerInternal() directly.

[dbasehore, 2016/04/08 20:38:11]

This is to handle the display state changing out from under the display
configurator (whether from powerd or some other place). I'll document this.

I assume SetDisplayPowerInternal will be called from the display configurator to
turn off the displays (and not save the off state in requested_power_state_)?

[Daniel Erat, 2016/04/08 20:42:11]

yeah, that's the idea. then we don't need to track an extra state --
requested_power_state_ contains the state that powerd asked for, and that's it.
the display can be configured differently for suspend, but we know what we want
to go back to after resuming.

+
   // True if |requested_power_state_| has been changed due to a user request.
   bool requested_power_state_change_;