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 <bradfitz@golang.org>
diff --git a/runtime/protoiface/methods.go b/runtime/protoiface/methods.go
index aeaf80a..32c04f6 100644
--- a/runtime/protoiface/methods.go
+++ b/runtime/protoiface/methods.go
@@ -23,7 +23,7 @@
 
 	// Size returns the size in bytes of the wire-format encoding of a message.
 	// Marshal must be provided if a custom Size is provided.
-	Size func(in SizeInput) SizeOutput
+	Size func(SizeInput) SizeOutput
 
 	// Marshal formats a message in the wire-format encoding to the provided buffer.
 	// Size should be provided if a custom Marshal is provided.