Pass root ServerWindow id to FrameGenerator.

services/ui/ws/frame_generator_unittest.cc

Index: services/ui/ws/frame_generator_unittest.cc
diff --git a/services/ui/ws/frame_generator_unittest.cc b/services/ui/ws/frame_generator_unittest.cc
index bd72df778dd92e5f07698e2f92e2383d2a4e67b7..659583efbaf68a0cd4add0d2d1db3ce921f90160 100644
--- a/services/ui/ws/frame_generator_unittest.cc
+++ b/services/ui/ws/frame_generator_unittest.cc
@@ -10,6 +10,7 @@
 #include "cc/quads/render_pass.h"
 #include "cc/quads/shared_quad_state.h"
 #include "services/ui/surfaces/display_compositor.h"
+#include "services/ui/ws/ids.h"
 #include "services/ui/ws/platform_display_init_params.h"
 #include "services/ui/ws/server_window.h"
 #include "services/ui/ws/server_window_compositor_frame_sink_manager.h"
@@ -22,6 +23,9 @@ namespace ws {
 namespace test {
 namespace {
 
+// Typical id for the display root ServerWindow.
+constexpr WindowId kRootDisplayId(0, 2);
+
 // Makes the window visible and creates the default surface for it.
 void InitWindow(ServerWindow* window) {
   window->SetVisible(true);
@@ -77,7 +81,7 @@ void FrameGeneratorTest::SetUp() {
       base::MakeUnique<ServerWindow>(&window_delegate_, WindowId()));
   PlatformDisplayInitParams init_params;
   frame_generator_ = base::MakeUnique<FrameGenerator>(
-      frame_generator_delegate_.get(), display_compositor_);
+      frame_generator_delegate_.get(), kRootDisplayId, display_compositor_);
   InitWindow(root_window());
 }