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

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

Index: ui/ozone/platform/drm/gpu/mock_scanout_buffer_generator.h
diff --git a/ui/ozone/platform/drm/gpu/mock_scanout_buffer_generator.h b/ui/ozone/platform/drm/gpu/mock_scanout_buffer_generator.h
index ecd8c933406276e240ad3a4ddfa42cb433741af1..fde77e3ba0f810d42827e2282467d50ac0f1e9b8 100644
--- a/ui/ozone/platform/drm/gpu/mock_scanout_buffer_generator.h
+++ b/ui/ozone/platform/drm/gpu/mock_scanout_buffer_generator.h
@@ -21,8 +21,14 @@ class MockScanoutBufferGenerator : public ScanoutBufferGenerator {
                                       uint32_t format,
                                       const gfx::Size& size) override;
 
+  void set_allocation_failure(bool allocation_failure) {
+    allocation_failure_ = allocation_failure;
+  }
+
  private:
   DISALLOW_COPY_AND_ASSIGN(MockScanoutBufferGenerator);
+
+  bool allocation_failure_ = false;
 };
 
 }  // namespace ui