By Idrees Ali and Phil Stewart WASHINGTON (Reuters) -The United States and Japan on Wednesday announced stepped-up security cooperation in the face of shared worries about China, and Washington strongly endorsed a major military buildup Tokyo announced last month. A joint statement issued after a meeting between their foreign and defense ministers in Washington said the two countries “provided a vision of a modernized Alliance postured to prevail in a new era of strategic competition.” “We agree that the PRC is the greatest shared strategic challenge that we and our allies and partners face,” …