Pass root ServerWindow id to FrameGenerator.

services/ui/ws/frame_generator.cc

Index: services/ui/ws/frame_generator.cc
diff --git a/services/ui/ws/frame_generator.cc b/services/ui/ws/frame_generator.cc
index d42f96d1143200fcc394bd94eccf228ecbb672ba..feedf2bd28a0cf5366a92b1afb203f3c13f7aa9f 100644
--- a/services/ui/ws/frame_generator.cc
+++ b/services/ui/ws/frame_generator.cc
@@ -24,10 +24,13 @@ namespace ws {
 
 FrameGenerator::FrameGenerator(
     FrameGeneratorDelegate* delegate,
+    ServerWindow* root_window,
     scoped_refptr<DisplayCompositor> display_compositor)
     : delegate_(delegate),
       display_compositor_(display_compositor),
-      frame_sink_id_(0, display_compositor->GenerateNextClientId()),
+      frame_sink_id_(
+          WindowIdToTransportId(root_window->id()),
+          static_cast<uint32_t>(mojom::CompositorFrameSinkType::DEFAULT)),
       draw_timer_(false, false),
       weak_factory_(this) {
   DCHECK(delegate_);