Add OnDisplayMetricsChanged in DisplayObserver.

ui/gfx/display_observer.h

Index: ui/gfx/display_observer.h
diff --git a/ui/gfx/display_observer.h b/ui/gfx/display_observer.h
index ca97247f9ede0a165208aa69810b65f1870c710d..84641a4cddb45e99ac22fee848a60af739b1ca34 100644
--- a/ui/gfx/display_observer.h
+++ b/ui/gfx/display_observer.h
@@ -15,8 +15,13 @@ class Display;
 // |DisplaySettingsProvier|. crbug.com/122863.
 class GFX_EXPORT DisplayObserver {
  public:
-  // Called when the |display|'s bound has changed.
-  virtual void OnDisplayBoundsChanged(const Display& display) = 0;
+  enum DisplayMetrics {
+    DISPLAY_METRICS_NONE = 0,
+    DISPLAY_METRICS_BOUNDS = 1 << 0,
+    DISPLAY_METRICS_WORK_AREA = 1 << 1,
+    DISPLAY_METRICS_DEVICE_SCALE_FACTOR = 1 << 2,
+    DISPLAY_METRICS_ROTATION = 1 << 3,
+  };
 
   // Called when |new_display| has been added.
   virtual void OnDisplayAdded(const Display& new_display) = 0;
@@ -24,6 +29,11 @@ class GFX_EXPORT DisplayObserver {
   // Called when |old_display| has been removed.
   virtual void OnDisplayRemoved(const Display& old_display) = 0;
 
+  // Called when a |display| has one or more metrics changed. |changed_metrics|
+  // will contain the information about the change, see |DisplayMetrics|.
+  virtual void OnDisplayMetricsChanged(
+      const Display& display, DisplayMetrics changed_metrics) = 0;
+
  protected:
   virtual ~DisplayObserver();
 };