ozone: Reject overlays when a test buffer can't be allocated.

ui/ozone/platform/drm/gpu/mock_scanout_buffer_generator.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/mock_scanout_buffer_generator.h"
 
 #include "ui/ozone/platform/drm/common/drm_util.h"
 #include "ui/ozone/platform/drm/gpu/mock_scanout_buffer.h"
 
 namespace ui {
 
 MockScanoutBufferGenerator::MockScanoutBufferGenerator() {}
 
 MockScanoutBufferGenerator::~MockScanoutBufferGenerator() {}
 
 scoped_refptr<ScanoutBuffer> MockScanoutBufferGenerator::Create(
     const scoped_refptr<DrmDevice>& drm,
     uint32_t format,
     const gfx::Size& size) {
+  if (allocation_failure_)
+    return nullptr;
+
   scoped_refptr<MockScanoutBuffer> buffer(new MockScanoutBuffer(size, format));
 
   return buffer;
 }
 
 }  // namespace ui