Pass root ServerWindow id to FrameGenerator.

services/ui/ws/platform_display.cc

Index: services/ui/ws/platform_display.cc
diff --git a/services/ui/ws/platform_display.cc b/services/ui/ws/platform_display.cc
index 56da720e51db63daecd262ccbc997ed4de70281b..2dd362a055a9ec9d72ac22bc940a9f2eca5e76c8 100644
--- a/services/ui/ws/platform_display.cc
+++ b/services/ui/ws/platform_display.cc
@@ -58,8 +58,9 @@ DefaultPlatformDisplay::DefaultPlatformDisplay(
 #if !defined(OS_ANDROID)
       cursor_loader_(ui::CursorLoader::Create()),
 #endif
-      frame_generator_(
-          new FrameGenerator(this, init_params.display_compositor)),
+      frame_generator_(new FrameGenerator(this,
+                                          init_params.root_window,
+                                          init_params.display_compositor)),
       metrics_(init_params.metrics) {
 }
 
@@ -83,7 +84,6 @@ void DefaultPlatformDisplay::Init(PlatformDisplayDelegate* delegate) {
 #else
   NOTREACHED() << "Unsupported platform";
 #endif
-  delegate_->CreateRootWindow(metrics_.bounds.size());
 
   platform_window_->SetBounds(bounds);
   platform_window_->Show();