platform-window: Add a PlatformWindowFactory for creating windows.

ui/aura/window_tree_host_win.cc

Index: ui/aura/window_tree_host_win.cc
diff --git a/ui/aura/window_tree_host_win.cc b/ui/aura/window_tree_host_win.cc
index 51bbcc6d7d55b7167ab8eedf52a79cd205740b2c..625776306fc7b103fdb9486f248e6068efa3d754 100644
--- a/ui/aura/window_tree_host_win.cc
+++ b/ui/aura/window_tree_host_win.cc
@@ -19,7 +19,8 @@
 #include "ui/gfx/insets.h"
 #include "ui/gfx/native_widget_types.h"
 #include "ui/gfx/screen.h"
-#include "ui/platform_window/win/win_window.h"
+#include "ui/platform_window/platform_window_factory.h"
+#include "ui/platform_window/types/platform_window.h"
 
 using std::max;
 using std::min;
@@ -45,7 +46,10 @@ gfx::Size WindowTreeHost::GetNativeScreenSize() {
 WindowTreeHostWin::WindowTreeHostWin(const gfx::Rect& bounds)
     : has_capture_(false),
       widget_(gfx::kNullAcceleratedWidget),
-      window_(new ui::WinWindow(this, bounds)) {
+      window_(ui::PlatformWindowFactory::GetInstance()->CreatePlatformWindow(
+          this,
+          bounds)) {
+  CHECK(window_);
 }
 
 WindowTreeHostWin::~WindowTreeHostWin() {