views: Extract Widget observer into its own header file.

ash/wm/app_list_controller.cc

Index: ash/wm/app_list_controller.cc
diff --git a/ash/wm/app_list_controller.cc b/ash/wm/app_list_controller.cc
index 9e409fd1d8adcb8e1e90a0f96d5dff014fb495f7..b7cc863f0c187898164a4098a48d7e0c8f6e2c11 100644
--- a/ash/wm/app_list_controller.cc
+++ b/ash/wm/app_list_controller.cc
@@ -21,6 +21,7 @@
 #include "ui/compositor/layer.h"
 #include "ui/compositor/scoped_layer_animation_settings.h"
 #include "ui/gfx/transform_util.h"
+#include "ui/views/widget/widget.h"
 
 namespace ash {
 namespace internal {
@@ -288,7 +289,7 @@ void AppListController::OnImplicitAnimationsCompleted() {
 }
 
 ////////////////////////////////////////////////////////////////////////////////
-// AppListController, views::Widget::Observer implementation:
+// AppListController, views::WidgetObserver implementation:
 
 void AppListController::OnWidgetClosing(views::Widget* widget) {
   DCHECK(view_->GetWidget() == widget);