third_party/protobuf: Update to HEAD (83d681ee2c)

third_party/protobuf/java/core/src/main/java/com/google/protobuf/DynamicMessage.java

Index: third_party/protobuf/java/core/src/main/java/com/google/protobuf/DynamicMessage.java
diff --git a/third_party/protobuf/java/core/src/main/java/com/google/protobuf/DynamicMessage.java b/third_party/protobuf/java/core/src/main/java/com/google/protobuf/DynamicMessage.java
index 859a9e8fc512e55c138c5a2939bd5edd40bd9743..c54da67fbb5ac06c56db3c7833bc493a5a12a73f 100644
--- a/third_party/protobuf/java/core/src/main/java/com/google/protobuf/DynamicMessage.java
+++ b/third_party/protobuf/java/core/src/main/java/com/google/protobuf/DynamicMessage.java
@@ -526,6 +526,14 @@ public final class DynamicMessage extends AbstractMessage {
           fields.clearField(oldField);
         }
         oneofCases[index] = field;
+      } else if (field.getFile().getSyntax() == Descriptors.FileDescriptor.Syntax.PROTO3) {
+        if (!field.isRepeated()
+            && field.getJavaType() != FieldDescriptor.JavaType.MESSAGE
+            && value.equals(field.getDefaultValue())) {
+          // In proto3, setting a field to its default value is equivalent to clearing the field.
+          fields.clearField(field);
+          return this;
+        }
       }
       fields.setField(field, value);
       return this;