internal/impl: update uses of deprecated ExtensionField methods

Change ExtensionField.GetType to ExtensionField.Type.

Change-Id: Ib1c933fc100848c2904fcff7cfda29a5162fe1f8
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/204799
Reviewed-by: Joe Tsai <joetsai@google.com>
diff --git a/internal/impl/codec_messageset.go b/internal/impl/codec_messageset.go
index 2493028..0b3746d 100644
--- a/internal/impl/codec_messageset.go
+++ b/internal/impl/codec_messageset.go
@@ -33,7 +33,7 @@
 		return 0
 	}
 	for _, x := range ext {
-		xi := mi.extensionFieldInfo(x.GetType())
+		xi := mi.extensionFieldInfo(x.Type())
 		if xi.funcs.size == nil {
 			continue
 		}
@@ -85,7 +85,7 @@
 }
 
 func marshalMessageSetField(mi *MessageInfo, b []byte, x ExtensionField, opts marshalOptions) ([]byte, error) {
-	xi := mi.extensionFieldInfo(x.GetType())
+	xi := mi.extensionFieldInfo(x.Type())
 	num, _ := wire.DecodeTag(xi.wiretag)
 	b = messageset.AppendFieldStart(b, num)
 	b, err := xi.funcs.marshal(b, x.Value(), wire.EncodeTag(messageset.FieldMessage, wire.BytesType), opts)