Support for C++ files generated by protobuf/grpc (#3640)

* Support for C++ files generated by protobuf/grpc

This changeset includes a sample generated file.

[grpc](http://grpc.io) is a high performance, open-source universal
RPC framework.

* Account for older gRPC protobuf plugin message

test/test_blob.rb

@@ -235,6 +235,10 @@ class TestBlob < Minitest::Test
 
     # Racc-generated Ruby
     assert sample_blob_memory("Ruby/racc.rb").generated?
+
+    # protobuf/grpc-plugin C++
+    assert sample_blob_memory("C++/hello.grpc.pb.h").generated?
+    assert sample_blob_memory("C++/grpc.pb.cc").generated?
   end
 
   def test_vendored