To enable Metal‘s validation layers in both ANGLE’s standalone test suites as well as in Chromium's GPU process, per this CL, set the following environment variables:
MTL_DEBUG_LAYER=1 MTL_DEBUG_LAYER_VALIDATE_LOAD_ACTIONS=1 MTL_DEBUG_LAYER_VALIDATE_STORE_ACTIONS=1 MTL_DEBUG_LAYER_VALIDATE_UNRETAINED_RESOURCES=4