Added alias getFieldProto3 as used by older generated code.
Added alias getFieldProto3 as used by older generated code.

Un-breaks users who have old generated code and upgrade
to the 3.1.0 release.

protocolbuffers/protobuf-javascript,protocolbuffers/protobuf-javascript,protocolbuffers/protobuf-javascript,protocolbuffers/protobuf-javascript