Add OnDisplayMetricsChanged in DisplayObserver.

ash/shelf/shelf_window_watcher.cc

Index: ash/shelf/shelf_window_watcher.cc
diff --git a/ash/shelf/shelf_window_watcher.cc b/ash/shelf/shelf_window_watcher.cc
index b16deeaf283c2265f703688079290a77db261c6b..1ad9f75dee74cc19b1862399d4fa8f75c7e54257 100644
--- a/ash/shelf/shelf_window_watcher.cc
+++ b/ash/shelf/shelf_window_watcher.cc
@@ -267,9 +267,6 @@ void ShelfWindowWatcher::OnWindowPropertyChanged(aura::Window* window,
   AddShelfItem(window);
 }
 
-void ShelfWindowWatcher::OnDisplayBoundsChanged(const gfx::Display& display) {
-}
-
 void ShelfWindowWatcher::OnDisplayAdded(const gfx::Display& new_display) {
   // Add a new RootWindow and its ActivationClient to observed list.
   aura::Window* root_window = Shell::GetInstance()->display_controller()->
@@ -288,4 +285,9 @@ void ShelfWindowWatcher::OnDisplayRemoved(const gfx::Display& old_display) {
   // Do nothing here.
 }
 
+void ShelfWindowWatcher::OnDisplayMetricsChanged(
+    const gfx::Display&,
+    DisplayObserver::DisplayMetrics) {
+}
+
 }  // namespace ash