firebase-ios-sdk

Convert grpc and protobuf CMake builds to add_subdirectory (#1537)

* Build zlib with grpc's add_subdirectory

* Build grpc and its children with add_subdirectory.

* Build c-ares with grpc's add_subdirectory

* Convert protobuf to a download-only ExternalProject

* Temporarily break protobuf within grpc

This works because libgrpc has no actual dependency on protobuf

* Update boringssl to master@{2018-07-10}