The country’s largest water district on Tuesday cemented support for Governor Jerry Brown’s contentious Bay Delta water project, agreeing to pay an estimated $4.3 billion, primarily through rate hikes on millions of Southern Californians. The Metropolitan Water District’s decision to fund up to 25.9 percent of the $17 billion California WaterFix, which would divert water from the West Coast’s largest estuary through two 40-foot-wide 35-mile-long tunnels, comes as a boost to a project beset by environmental and financial uncertainties.