Pass root ServerWindow id to FrameGenerator.

services/ui/ws/test_utils.cc

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "services/ui/ws/test_utils.h"
 
 #include <utility>
 
 #include "base/memory/ptr_util.h"
 #include "cc/output/copy_output_request.h"
@@ skipping 23 matching lines @@
                                mojom::Cursor* cursor_storage)
       : id_(id),
         is_primary_(is_primary),
         cursor_storage_(cursor_storage) {
     display_metrics_.bounds = gfx::Rect(0, 0, 400, 300);
     display_metrics_.device_scale_factor = 1.f;
   }
   ~TestPlatformDisplay() override {}
 
   // PlatformDisplay:
-  void Init(PlatformDisplayDelegate* delegate) override {
-    delegate->CreateRootWindow(display_metrics_.bounds.size());
-  }
+  void Init(PlatformDisplayDelegate* delegate) override {}
   int64_t GetId() const override { return id_; }
   void SchedulePaint(const ServerWindow* window,
                      const gfx::Rect& bounds) override {}
   void SetViewportSize(const gfx::Size& size) override {}
   void SetTitle(const base::string16& title) override {}
   void SetCapture() override {}
   void ReleaseCapture() override {}
   void SetCursorById(mojom::Cursor cursor) override {
     *cursor_storage_ = cursor;
   }
@@ skipping 553 matching lines @@
     return nullptr;
   if (!tree->AddWindow(parent_client_id, client_window_id))
     return nullptr;
   *client_id = client_window_id;
   return tree->GetWindowByClientId(client_window_id);
 }
 
 }  // namespace test
 }  // namespace ws
 }  // namespace ui