runtime/protoiface: trivial change to Size
runtime/protoiface: trivial change to Size

Remove named input argument to be consistent with other methods.

Change-Id: I22c48abf76e007a1319bfb037bbb2b7bfb66cee7
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/220688
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

protocolbuffers/protobuf-go,protocolbuffers/protobuf-go