The top Democrat and Republican on the Senate Banking Committee said they had reached agreement on the broad outlines of a bill to overhaul Fannie Mae and Freddie Mac and the nation’s $9.9 trillion mortgage market, drawing a strong statement of support from the White House. The bill will call for replacing Fannie and Freddie with a new system of federally insured mortgage securities in which private insurers would be required to take initial losses before any government guarantee would be triggered.