Add OnDisplayMetricsChanged in DisplayObserver.

ash/shell/window_watcher.cc

Index: ash/shell/window_watcher.cc
diff --git a/ash/shell/window_watcher.cc b/ash/shell/window_watcher.cc
index 55231cf7a0608148a22974e4fc43f7f635f64e4a..a934af18c90c85eb32b62ee037864fc06a1bb390 100644
--- a/ash/shell/window_watcher.cc
+++ b/ash/shell/window_watcher.cc
@@ -138,9 +138,6 @@ void WindowWatcher::OnWillRemoveWindow(aura::Window* window) {
   }
 }
 
-void WindowWatcher::OnDisplayBoundsChanged(const gfx::Display& display) {
-}
-
 void WindowWatcher::OnDisplayAdded(const gfx::Display& new_display) {
   aura::Window* root = Shell::GetInstance()->display_controller()->
       GetRootWindowForDisplayId(new_display.id());
@@ -152,5 +149,9 @@ void WindowWatcher::OnDisplayRemoved(const gfx::Display& old_display) {
   // remove observers.
 }
 
+void WindowWatcher::OnDisplayMetricsChanged(const gfx::Display&,
+                                            DisplayObserver::DisplayMetrics) {
+}
+
 }  // namespace shell
 }  // namespace ash