Introdcue AshWindowTreeHost and move ash/chrome specific code in WTH to ash.

chrome/browser/ui/ash/chrome_shell_delegate.cc

Index: chrome/browser/ui/ash/chrome_shell_delegate.cc
diff --git a/chrome/browser/ui/ash/chrome_shell_delegate.cc b/chrome/browser/ui/ash/chrome_shell_delegate.cc
index 8a3aacd93e7a6437781e6a9dd784e84425878ceb..2307885cfe0d0995e8c5e46cefb302842635015f 100644
--- a/chrome/browser/ui/ash/chrome_shell_delegate.cc
+++ b/chrome/browser/ui/ash/chrome_shell_delegate.cc
@@ -7,7 +7,6 @@
 #include "apps/app_window.h"
 #include "apps/app_window_registry.h"
 #include "ash/content_support/gpu_support_impl.h"
-#include "ash/host/window_tree_host_factory.h"
 #include "ash/magnifier/magnifier_constants.h"
 #include "ash/wm/window_state.h"
 #include "ash/wm/window_util.h"
@@ -144,10 +143,6 @@ ui::MenuModel* ChromeShellDelegate::CreateContextMenu(
   return new LauncherContextMenu(shelf_delegate_, root);
 }
 
-ash::WindowTreeHostFactory* ChromeShellDelegate::CreateWindowTreeHostFactory() {
-  return ash::WindowTreeHostFactory::Create();
-}
-
 ash::GPUSupport* ChromeShellDelegate::CreateGPUSupport() {
   // Chrome uses real GPU support.
   return new ash::GPUSupportImpl;