views: Extract Widget observer into its own header file.

ui/views/widget/widget_unittest.cc

Index: ui/views/widget/widget_unittest.cc
diff --git a/ui/views/widget/widget_unittest.cc b/ui/views/widget/widget_unittest.cc
index 7260c2be7f1c4d23575cf50da05e6313b32c78e7..46fb438f76698da47fc43ec3f981c8b49eb7df0c 100644
--- a/ui/views/widget/widget_unittest.cc
+++ b/ui/views/widget/widget_unittest.cc
@@ -610,7 +610,7 @@ TEST_F(WidgetOwnershipTest,
 //
 
 class WidgetObserverTest : public WidgetTest,
-                                  Widget::Observer {
+                           public WidgetObserver {
  public:
   WidgetObserverTest()
       : active_(NULL),
@@ -622,6 +622,7 @@ class WidgetObserverTest : public WidgetTest,
 
   virtual ~WidgetObserverTest() {}
 
+  // Overridden from WidgetObserver:
   virtual void OnWidgetClosing(Widget* widget) OVERRIDE {
     if (active_ == widget)
       active_ = NULL;