Adds a factory function to ViewsDelegate for DesktopWindowTreeHost

ui/views/widget/desktop_aura/desktop_native_widget_aura.cc

Index: ui/views/widget/desktop_aura/desktop_native_widget_aura.cc
diff --git a/ui/views/widget/desktop_aura/desktop_native_widget_aura.cc b/ui/views/widget/desktop_aura/desktop_native_widget_aura.cc
index 2e903f9b6ad3fbbc4027c6dfa112c61014850ee5..edeae4a89a72f8149dba046b07ac83093ae5cde3 100644
--- a/ui/views/widget/desktop_aura/desktop_native_widget_aura.cc
+++ b/ui/views/widget/desktop_aura/desktop_native_widget_aura.cc
@@ -31,6 +31,7 @@
 #include "ui/views/corewm/tooltip_controller.h"
 #include "ui/views/drag_utils.h"
 #include "ui/views/view_constants_aura.h"
+#include "ui/views/views_delegate.h"
 #include "ui/views/widget/desktop_aura/desktop_capture_client.h"
 #include "ui/views/widget/desktop_aura/desktop_event_client.h"
 #include "ui/views/widget/desktop_aura/desktop_focus_rules.h"
@@ -415,10 +416,20 @@ void DesktopNativeWidgetAura::InitNativeWidget(
   wm::SetShadowElevation(content_window_, wm::ShadowElevation::NONE);
 
   if (!desktop_window_tree_host_) {
-    desktop_window_tree_host_ =
-        params.desktop_window_tree_host
-            ? params.desktop_window_tree_host
-            : DesktopWindowTreeHost::Create(native_widget_delegate_, this);
+    if (params.desktop_window_tree_host) {
+      desktop_window_tree_host_ = params.desktop_window_tree_host;
+    } else if (!ViewsDelegate::GetInstance()
+                    ->desktop_window_tree_host_factory()
+                    .is_null()) {
+      desktop_window_tree_host_ =
+          ViewsDelegate::GetInstance()
+              ->desktop_window_tree_host_factory()
+              .Run(params, native_widget_delegate_, this)
+              .release();
+    } else {
+      desktop_window_tree_host_ =
+          DesktopWindowTreeHost::Create(native_widget_delegate_, this);
+    }
     host_.reset(desktop_window_tree_host_->AsWindowTreeHost());
   }
   desktop_window_tree_host_->Init(content_window_, params);