In a significant boost for Gov. Jerry Brown’s $17 billion plan to build two massive tunnels under the Sacramento-San Joaquin Valley Delta to more easily move water south, Silicon Valley’s largest water agency Tuesday endorsed the project and voted to commit up to $650 million to help pay for it. With a 4-3 vote after a packed four-hour meeting, the Santa Clara Valley Water District reversed a decision it made in October to oppose the two-tunnel project.