Google co-founder Sergey Brin is backing a pair of ballot initiatives that oppose California’s controversial proposed billionaire tax in a high-stakes showdown ahead of the November election. The two counter-measures — aimed as scuttling a proposed 5% levy on residents with assets exceeding $1 billion — would prohibit new taxes on a person’s financial assets and increase audits of programs funded by such taxes. FilmMagic The group that authored the initiatives, Building a Better California, has benefitted from $57 million in donations from Brin along with $36 million from other billionaires and business leaders, The Wall Street Journal reported. The first measure, which has collected about 1.4 million signatures, would prohibit new taxes on personal property, including retirement accounts, intellectual property and financial assets, while excluding real estate. It would also prevent those types of taxes from being applied retroactively.…