third_party/protobuf: Update to HEAD (f52e188fe4)

third_party/protobuf/js/commonjs/export_testdeps.js

 /**
  * @fileoverview Export symbols needed by tests in CommonJS style.
  *
  * This file is like export.js, but for symbols that are only used by tests.
  * However we exclude assert functions here, because they are exported into
  * the global namespace, so those are handled as a special case in
  * export_asserts.js.
  */
 
 goog.require('goog.crypt.base64');
 goog.require('jspb.arith.Int64');
 goog.require('jspb.arith.UInt64');
 goog.require('jspb.BinaryEncoder');
 goog.require('jspb.BinaryDecoder');
+goog.require('jspb.BinaryWriter');
 goog.require('jspb.utils');
 
 exports.goog = goog;
 exports.jspb = jspb;