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/message_reflect.go b/internal/impl/message_reflect.go
index 594ed9e..bc50303 100644
--- a/internal/impl/message_reflect.go
+++ b/internal/impl/message_reflect.go
@@ -145,7 +145,7 @@
 func (m *extensionMap) Range(f func(pref.FieldDescriptor, pref.Value) bool) {
 	if m != nil {
 		for _, x := range *m {
-			xt := x.GetType()
+			xt := x.Type()
 			if !f(xt.TypeDescriptor(), x.Value()) {
 				return
 			}