WASHINGTON — Facing hundreds of millions of dollars in lawsuits, the giant biotechnology company Monsanto last year received a legislative gift from the House of Representatives, a one-paragraph addition to a sweeping chemical safety bill that could help shield it from legal liability for a toxic chemical only it made. Monsanto insists it did not ask for the addition. House aides deny it is a gift at all. But the provision would benefit the only manufacturer in the United States of now-banned polychlorinated biphenyls, chemicals known as PCBs, a mainstay of Monsanto sales for decades. The PCB provision is one of several sticking points that negotiators must finesse before Congress can pass a law to revamp the way thousands of chemicals are regulated in the United States.