code: function ReadOnly(properties) {
                          if (properties)
                              for (var keys = Object.keys(properties), i = 0; i < keys.length; ++i)
                                  if (properties[keys[i]] != null)
                                      this[keys[i]] = properties[keys[i]];
                      }
docs: Properties of a ReadOnly.
  @memberof google.firestore.v1.TransactionOptions
  @interface IReadOnly
  @property {google.protobuf.ITimestamp|null} [readTime] ReadOnly readTime
   
  Constructs a new ReadOnly.
  @memberof google.firestore.v1.TransactionOptions
  @classdesc Represents a ReadOnly.
  @implements IReadOnly
  @constructor
  @param {google.firestore.v1.TransactionOptions.IReadOnly=} [properties] Properties to set