views/mus: Fix some flaky crashes during test teardown.

ui/views/mus/surface_context_factory.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/views/mus/surface_context_factory.h"
 
 #include "base/memory/ptr_util.h"
 #include "cc/output/output_surface.h"
 #include "cc/resources/shared_bitmap_manager.h"
 #include "cc/surfaces/surface_id_allocator.h"
@@ skipping 21 matching lines @@
     mus::Window* window,
     mus::mojom::SurfaceType surface_type)
     : surface_binding_(connector, window, surface_type),
       next_surface_id_namespace_(1u) {}
 
 SurfaceContextFactory::~SurfaceContextFactory() {}
 
 void SurfaceContextFactory::CreateOutputSurface(
     base::WeakPtr<ui::Compositor> compositor) {
   // NOTIMPLEMENTED();
-  compositor->SetOutputSurface(surface_binding_.CreateOutputSurface());
+  std::unique_ptr<cc::OutputSurface> surface =
+      surface_binding_.CreateOutputSurface();
+  if (surface)
+    compositor->SetOutputSurface(std::move(surface));
 }
 
 std::unique_ptr<ui::Reflector> SurfaceContextFactory::CreateReflector(
     ui::Compositor* mirroed_compositor,
     ui::Layer* mirroring_layer) {
   // NOTIMPLEMENTED();
   return base::WrapUnique(new FakeReflector);
 }
 
 void SurfaceContextFactory::RemoveReflector(ui::Reflector* reflector) {
@@ skipping 44 matching lines @@
   // NOTIMPLEMENTED();
   return nullptr;
 }
 
 void SurfaceContextFactory::ResizeDisplay(ui::Compositor* compositor,
                                           const gfx::Size& size) {
   // NOTIMPLEMENTED();
 }
 
 }  // namespace views