They can't do it even if capitalisation of Google drops to $1 - everyone seems to have forgotten about peculiar way shareholding is structured in Google - the founders have full control even though they don't have majority of shares. So buying 100% shares on the open market won't result in control of the company.
Owning majority shares does not give you full control of a company. What gives you the control is the contracts. Majority shareholders get full control by default if there are no contracts to explicitly say who has control.