exo: Use aura::Windows instead of views::Views as exo::Surfaces.

components/exo/shell_surface.h

Index: components/exo/shell_surface.h
diff --git a/components/exo/shell_surface.h b/components/exo/shell_surface.h
index 150d32b6734f65c3633fe6cd1b4054d7719f1154..7a2c189af26bc3fa9a48c824b911f1ee57b04b05 100644
--- a/components/exo/shell_surface.h
+++ b/components/exo/shell_surface.h
@@ -28,7 +28,8 @@ class Surface;
 // metadata like title and class, etc.
 class ShellSurface : public SurfaceDelegate,
                      public SurfaceObserver,
-                     public views::WidgetDelegate {
+                     public views::WidgetDelegate,
+                     public views::View {
  public:
   explicit ShellSurface(Surface* surface);
   ~ShellSurface() override;
@@ -66,6 +67,9 @@ class ShellSurface : public SurfaceDelegate,
   views::NonClientFrameView* CreateNonClientFrameView(
       views::Widget* widget) override;
 
+  // Overridden from views::View:
+  gfx::Size GetPreferredSize() const override;
+
  private:
   scoped_ptr<views::Widget> widget_;
   Surface* surface_;