ash: Add RootWindowHostFactory class.

ui/aura/root_window.cc

Index: ui/aura/root_window.cc
diff --git a/ui/aura/root_window.cc b/ui/aura/root_window.cc
index 6e111561f4fe116ef29eef89a26690c53209248f..268aecf70b208ea668ff7bf01fb8ea8604171f2a 100644
--- a/ui/aura/root_window.cc
+++ b/ui/aura/root_window.cc
@@ -20,6 +20,7 @@
 #include "ui/aura/client/focus_client.h"
 #include "ui/aura/client/screen_position_client.h"
 #include "ui/aura/env.h"
+#include "ui/aura/root_window_delegate.h"
 #include "ui/aura/root_window_host.h"
 #include "ui/aura/root_window_observer.h"
 #include "ui/aura/window.h"
@@ -75,8 +76,9 @@ void SetLastMouseLocation(const Window* root_window,
 
 RootWindowHost* CreateHost(RootWindow* root_window,
                            const RootWindow::CreateParams& params) {
-  RootWindowHost* host = params.host ?
-      params.host : RootWindowHost::Create(params.initial_bounds);
+  RootWindowHost* host = params.delegate ?
+      params.delegate->CreateRootWindowHost(params.initial_bounds) :
+      RootWindowHost::Create(params.initial_bounds);
   host->SetDelegate(root_window);
   return host;
 }
@@ -85,7 +87,7 @@ RootWindowHost* CreateHost(RootWindow* root_window,
 
 RootWindow::CreateParams::CreateParams(const gfx::Rect& a_initial_bounds)
     : initial_bounds(a_initial_bounds),
-      host(NULL) {
+      delegate(NULL) {
 }
 
 ////////////////////////////////////////////////////////////////////////////////
@@ -113,6 +115,7 @@ RootWindow::RootWindow(const CreateParams& params)
       ALLOW_THIS_IN_INITIALIZER_LIST(held_mouse_event_factory_(this)) {
   SetName("RootWindow");
 
+  delegate_.reset(params.delegate);
   compositor_.reset(new ui::Compositor(this, host_->GetAcceleratedWidget()));
   DCHECK(compositor_.get());
   compositor_->AddObserver(this);