Example for use with protocol buffers

**Is your feature request related to a problem? Please describe.**
We use Protobuf objects extensively in our stack, including protobuf-js objects. We're wondering what the best way would be to plugin with protobuf-js, from J2CL.

**Describe the solution you'd like**
An example, perhaps in the JS Interop Cookbook, of how one would go about using protobuf objects in J2CL effectively.

**Describe alternatives you've considered**
1) We've thought about using the Java proto objects, but we have no idea if those will convert properly, or be as tightly optimized as protobuf-js is.

2) Otherwise, we could write bindings to call into the protobuf objects (for example, [like this](https://github.com/google/j2cl/blob/master/docs/jsinterop-by-example.md#jstype---interface-with-closure-utilities)), but the surface area of those bindings would be huge and hard to maintain.

3) We've thought about writing a protoc plugin to generate these files, but that's beyond the scope of our team's skills right now