chromecast: simplify the generated protobuf code dependencies
- build only the .pb.cc file, the .pb.h file will be generated at the same time so will not need a build rule when it's time to build it, as long as it needs the .pb.cc to be build before.
- don't use a dummy .chromecast_proto which was used to group the .pb.cc and .pb.h we just generate the .pb.cc. The .pb.h is just a by-product.
Now the .pb.cc is directly tied to the cast_channel.proto files changes and the PROTOBUF_VERSION value changes.
Alternative to !2827 (closed)
This still addresses the issue raised in !2787 (comment 352815) where protoc would be called twice.