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

ash/shell/shell_delegate_impl.cc

Index: ash/shell/shell_delegate_impl.cc
diff --git a/ash/shell/shell_delegate_impl.cc b/ash/shell/shell_delegate_impl.cc
index dfae0939d5ae10f18f59553e18431378a55e8dd7..840c6bb0a3ade0967ee5ed3a28db8fe74e190ef6 100644
--- a/ash/shell/shell_delegate_impl.cc
+++ b/ash/shell/shell_delegate_impl.cc
@@ -8,7 +8,6 @@
 #include "ash/default_accessibility_delegate.h"
 #include "ash/default_user_wallpaper_delegate.h"
 #include "ash/gpu_support_stub.h"
-#include "ash/host/window_tree_host_factory.h"
 #include "ash/media_delegate.h"
 #include "ash/new_window_delegate.h"
 #include "ash/session_state_delegate.h"
@@ -172,10 +171,6 @@ ui::MenuModel* ShellDelegateImpl::CreateContextMenu(
   return new ContextMenu(root);
 }
 
-WindowTreeHostFactory* ShellDelegateImpl::CreateWindowTreeHostFactory() {
-  return WindowTreeHostFactory::Create();
-}
-
 GPUSupport* ShellDelegateImpl::CreateGPUSupport() {
   // Real GPU support depends on src/content, so just use a stub.
   return new GPUSupportStub;