chromeos: Ignore power button presses when screen is off.

ash/wm/power_button_controller.h

Index: ash/wm/power_button_controller.h
diff --git a/ash/wm/power_button_controller.h b/ash/wm/power_button_controller.h
index 40719d8bd046eb049b2306b80ccd9cf2f9e7c8d4..efe5f53544bd65617f4aadcba86935b675bb95de 100644
--- a/ash/wm/power_button_controller.h
+++ b/ash/wm/power_button_controller.h
@@ -142,6 +142,9 @@ class ASH_EXPORT PowerButtonController : public aura::RootWindowObserver,
     has_legacy_power_button_ = legacy;
   }
 
+  // Called when the current screen brightness changes.
+  void OnScreenBrightnessChanged(double percent);
+
   // Called when Chrome gets a request to display the lock screen.
   void OnStartingLock();
 
@@ -203,6 +206,9 @@ class ASH_EXPORT PowerButtonController : public aura::RootWindowObserver,
   bool power_button_down_;
   bool lock_button_down_;
 
+  // Is the screen currently turned off?
+  bool screen_is_off_;
+
   // Are we in the process of shutting the machine down?
   bool shutting_down_;