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

ash/shell_delegate.h

Index: ash/shell_delegate.h
diff --git a/ash/shell_delegate.h b/ash/shell_delegate.h
index eaca17717a375d0bfde2c07b4cc34875040140df..d41132b5f78e15aea6a50dccd126f28fce9a8102 100644
--- a/ash/shell_delegate.h
+++ b/ash/shell_delegate.h
@@ -42,7 +42,6 @@ namespace ash {
 class AccessibilityDelegate;
 class MediaDelegate;
 class NewWindowDelegate;
-class WindowTreeHostFactory;
 class SessionStateDelegate;
 class ShelfDelegate;
 class ShelfItemDelegate;
@@ -146,10 +145,6 @@ class ASH_EXPORT ShellDelegate {
       ash::ShelfItemDelegate* item_delegate,
       ash::ShelfItem* item) = 0;
 
-  // Creates a root window host factory. Shell takes ownership of the returned
-  // value.
-  virtual WindowTreeHostFactory* CreateWindowTreeHostFactory() = 0;
-
   // Creates a GPU support object. Shell takes ownership of the object.
   virtual GPUSupport* CreateGPUSupport() = 0;