Starbucks signed with Square Inc. that will have the world’s biggest coffee chain use the 2-year old startup’s technology to process mobile payments. It has won over other established payment processors. The deal was announced Wednesday. According to the announcement, Starbucks will invest $25 million in Square, which is a San Francisco-based company led by Jack Dorsey. Dorsey is the co-founder o social media company Twitter. Starbucks CEO Howard Schultz will be part of Square’s board. Starbucks...