Oracle completed its acquisition of UK adtech startup Grapeshot last week in a deal that sources said was worth $325 million. Grapeshot's technology helps brands avoid unwittingly showing ads next to embarrassing content, like airlines advertising next to news stories about plane crashes. The deal has turned Grapeshot's executive team into millionaires, and is another big Cambridge success, according to its investors. But they also said Grapeshot could have one day been worth $1 billion, and that the company fought off investment offers from private equity firms.