A high-stakes battle is brewing over California’s proposed “billionaire tax,” with Silicon Valley heavyweights already pouring millions into efforts to stop it.Sergey Brin, the billionaire co-founder of Google, has committed roughly $45 million to a group opposing the measure called Building a Better California, according to the Wall Street Journal.In total, Building a Better CA has raised about $80 million to fight the tax — funding campaigns, ads and ballot efforts.The initiative would impose a one-time 5% tax on individuals