More than six months ago, Google approached WhatsApp with an odd offer: It would pay the mobile messaging startup in exchange for the right to be notified if the messaging app entered into acquisition talks with other companies, according to a person involved in the process and two people briefed on it. WhatsApp rebuffed the offer, which could have been worth millions of dollars, according to the person involved in the process. A number of months and a quick 11-day negotiation later, WhatsApp and its 450 million active monthly users ended up in the hands of Google’s archrival, Facebook. And Google, which had repeatedly expressed interested in buying WhatsApp, had lost.