Update //third_party/protobuf to version 3.

third_party/protobuf/src/google/protobuf/source_context.proto

 // Protocol Buffers - Google's data interchange format
 // Copyright 2008 Google Inc.  All rights reserved.
-// http://code.google.com/p/protobuf/
+// https://developers.google.com/protocol-buffers/
 //
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 // notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 // copyright notice, this list of conditions and the following disclaimer
 // in the documentation and/or other materials provided with the
 // distribution.
 //     * Neither the name of Google Inc. nor the names of its
 // contributors may be used to endorse or promote products derived from
 // this software without specific prior written permission.
 //
 // THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 // "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 // LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
 // A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
 // OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 // SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
 // LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 // DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 // THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-// Author: kenton@google.com (Kenton Varda)
-//
-// This is like unittest_import.proto but with optimize_for = LITE_RUNTIME.
+syntax = "proto3";
 
-package protobuf_unittest_import;
+package google.protobuf;
 
-option optimize_for = LITE_RUNTIME;
+option csharp_namespace = "Google.Protobuf.WellKnownTypes";
+option java_package = "com.google.protobuf";
+option java_outer_classname = "SourceContextProto";
+option java_multiple_files = true;
+option java_generate_equals_and_hash = true;
+option objc_class_prefix = "GPB";
 
-option java_package = "com.google.protobuf";
-
-import public "google/protobuf/unittest_import_public_lite.proto";
-
-message ImportMessageLite {
-  optional int32 d = 1;
+// `SourceContext` represents information about the source of a
+// protobuf element, like the file in which it is defined.
+message SourceContext {
+  // The path-qualified name of the .proto file that contained the associated
+  // protobuf element.  For example: `"google/protobuf/source.proto"`.
+  string file_name = 1;
 }
-
-enum ImportEnumLite {
-  IMPORT_LITE_FOO = 7;
-  IMPORT_LITE_BAR = 8;
-  IMPORT_LITE_BAZ = 9;
-}