cirq-google: Export Serializer on top-level cirq_google namespace (#4474)
cirq-google: Export Serializer on top-level cirq_google namespace (#4474)

Review: a54b34aa100515ed7ce806c321d318eb5b457949@vtomole 
quantumlib/Cirq,quantumlib/Cirq,quantumlib/Cirq,quantumlib/Cirq,quantumlib/Cirq