Rename Size_ to Size

Previouslty "Size" was reserved by protoc-gen-gogoctrd and user-generated
"Size" was automatically renamed to "Size_" to avoid conflicts.

Signed-off-by: Kazuyoshi Kato <katokazu@amazon.com>

content/proxy/content_reader.go

@@ -38,7 +38,7 @@ func (ra *remoteReaderAt) ReadAt(p []byte, off int64) (n int, err error) {
 	rr := &contentapi.ReadContentRequest{
 		Digest: ra.digest.String(),
 		Offset: off,
-		Size_:  int64(len(p)),
+		Size:   int64(len(p)),
 	}
 	// we need a child context with cancel, or the eventually called
 	// grpc.NewStream will leak the goroutine until the whole thing is cleared.