WASHINGTON – A Senate panel put a top Google executive on the defensive Tuesday over the company’s powerful position in online advertising as some lawmakers look hopefully toward an expected antitrust case against the tech giant by the Trump administration.Donald Harrison, Google’s president for global partnerships and corporate development, insisted at a hearing by the Senate Judiciary antitrust subcommittee that Google’s ad business faces ample competition, has benefited consumers, and has kept prices low for advertisers and publishers such as local newspapers.The Justice Department has pursued a sweeping antitrust investigation of big tech companies, looking at whether the online platforms of Google, Facebook, Amazon and Apple have hurt.