DocxAttributeOutput::TableBackgrounds: use unique_ptr
DocxAttributeOutput::TableBackgrounds: use unique_ptr

Change-Id: I312d3d94c54f9a8537e2d22d347a75bd40360b80

JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core