Update //third_party/protobuf to version 3.

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

Index: third_party/protobuf/src/google/protobuf/source_context.proto
diff --git a/third_party/protobuf/src/google/protobuf/unittest_import_lite.proto b/third_party/protobuf/src/google/protobuf/source_context.proto
similarity index 72%
copy from third_party/protobuf/src/google/protobuf/unittest_import_lite.proto
copy to third_party/protobuf/src/google/protobuf/source_context.proto
index 81b117fe8489ecc4cba5410dbf876b4ca473657b..d76252ca841edba2598636ef122f264432edce73 100644
--- a/third_party/protobuf/src/google/protobuf/unittest_import_lite.proto
+++ b/third_party/protobuf/src/google/protobuf/source_context.proto
@@ -1,6 +1,6 @@
 // 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
@@ -28,24 +28,21 @@
 // (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.
-
-package protobuf_unittest_import;
+syntax = "proto3";
 
-option optimize_for = LITE_RUNTIME;
+package google.protobuf;
 
+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";
 
-import public "google/protobuf/unittest_import_public_lite.proto";
-
-message ImportMessageLite {
-  optional int32 d = 1;
-}
-
-enum ImportEnumLite {
-  IMPORT_LITE_FOO = 7;
-  IMPORT_LITE_BAR = 8;
-  IMPORT_LITE_BAZ = 9;
+// `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;
 }