Update //third_party/protobuf to version 3.

third_party/protobuf/objectivec/google/protobuf/SourceContext.pbobjc.h

+// Generated by the protocol buffer compiler.  DO NOT EDIT!
+// source: google/protobuf/source_context.proto
+
+#import "GPBProtocolBuffers.h"
+
+#if GOOGLE_PROTOBUF_OBJC_GEN_VERSION != 30000
+#error This file was generated by a different version of protoc which is incompatible with your Protocol Buffer library sources.
+#endif
+
+// @@protoc_insertion_point(imports)
+
+CF_EXTERN_C_BEGIN
+
+NS_ASSUME_NONNULL_BEGIN
+
+#pragma mark - GPBSourceContextRoot
+
+@interface GPBSourceContextRoot : GPBRootObject
+
+// The base class provides:
+//   + (GPBExtensionRegistry *)extensionRegistry;
+// which is an GPBExtensionRegistry that includes all the extensions defined by
+// this file and all files that it depends on.
+
+@end
+
+#pragma mark - GPBSourceContext
+
+typedef GPB_ENUM(GPBSourceContext_FieldNumber) {
+  GPBSourceContext_FieldNumber_FileName = 1,
+};
+
+// `SourceContext` represents information about the source of a
+// protobuf element, like the file in which it is defined.
+@interface GPBSourceContext : GPBMessage
+
+// The path-qualified name of the .proto file that contained the associated
+// protobuf element.  For example: `"google/protobuf/source.proto"`.
+@property(nonatomic, readwrite, copy, null_resettable) NSString *fileName;
+
+@end
+
+NS_ASSUME_NONNULL_END
+
+CF_EXTERN_C_END
+
+// @@protoc_insertion_point(global_scope)