TOKYO — President Ferdinand Marcos Jr. said Wednesday that his five-day visit here is expected to forge closer ties between the Philippines and Japan in ensuring security in the Asia-Pacific region.   Asked by reporters how he wants to improve cooperation with Japan in the South China Sea, Marcos said that “Japan wants to develop more and make our relationship even more robust and closer, especially in that regard.”   “This is a new element to our relationship because we are now talking about the security of the region,” the President said in an interview with reporters while aboard a flight to Japan.   Marcos said the Philippines’ dealings with Japan usually involve government-to-government, aid, grants,    In his pre-departure speech in Manila, the chief executive said his visit is part of a larger foreign policy agenda to forge closer political ties, stronger defense and security cooperation, as well as lasting economic partnerships with major countries in the region.