Make ozone/drm/mojo more immune to startup races

ui/ozone/platform/drm/gpu/drm_thread.cc

 // Copyright 2015 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 "ui/ozone/platform/drm/gpu/drm_thread.h"
 
 #include <gbm.h>
 #include <utility>
 
 #include "base/command_line.h"
@@ skipping 189 matching lines @@
 
 void DrmThread::SetWindowBounds(gfx::AcceleratedWidget widget,
                                 const gfx::Rect& bounds) {
   screen_manager_->GetWindow(widget)->SetBounds(bounds);
 }
 
 void DrmThread::SetCursor(const gfx::AcceleratedWidget& widget,
                           const std::vector<SkBitmap>& bitmaps,
                           const gfx::Point& location,
                           int32_t frame_delay_ms) {
-  screen_manager_->GetWindow(widget)
-      ->SetCursor(bitmaps, location, frame_delay_ms);
+  auto window = screen_manager_->GetWindow(widget);

[dnicoara, 2017/05/26 15:51:04]

Why is this needed? The window lifetime is "managed" by the browser process via
CreateWindow()/DestroyWindow(). It should never be the case that a window event
arrives before the window is created or after the window is destroyed.

+  if (!window)
+    return;
+  window->SetCursor(bitmaps, location, frame_delay_ms);
 }
 
 void DrmThread::MoveCursor(const gfx::AcceleratedWidget& widget,
                            const gfx::Point& location) {
   screen_manager_->GetWindow(widget)->MoveCursor(location);
 }
 
 void DrmThread::CheckOverlayCapabilities(
     gfx::AcceleratedWidget widget,
     const std::vector<OverlayCheck_Params>& overlays,
@@ skipping 68 matching lines @@
                                        correction_matrix);
 }
 
 // DrmThread requires a BindingSet instead of a simple Binding because it will
 // be used from multiple threads in multiple processes.
 void DrmThread::AddBinding(ozone::mojom::DeviceCursorRequest request) {
   bindings_.AddBinding(this, std::move(request));
 }
 
 }  // namespace ui