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

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

Index: ui/ozone/platform/drm/gpu/mock_scanout_buffer_generator.cc
diff --git a/ui/ozone/platform/drm/gpu/mock_scanout_buffer_generator.cc b/ui/ozone/platform/drm/gpu/mock_scanout_buffer_generator.cc
index aa2dd0fd7f7ce54e62a40467de3be4d83a7e657e..ccf53a34ba584b6bc871e33aa50c5ec3bc7629a0 100644
--- a/ui/ozone/platform/drm/gpu/mock_scanout_buffer_generator.cc
+++ b/ui/ozone/platform/drm/gpu/mock_scanout_buffer_generator.cc
@@ -17,6 +17,9 @@ 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;