Protobuf for Traffic Annotation and first use by a URLFetcher.

net/traffic_annotation/network_traffic_annotation.h

Index: net/traffic_annotation/network_traffic_annotation.h
diff --git a/net/traffic_annotation/network_traffic_annotation.h b/net/traffic_annotation/network_traffic_annotation.h
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+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef NET_TRAFFIC_ANNOTATION_NETWORK_TRAFFIC_ANNOTATION_H_
+#define NET_TRAFFIC_ANNOTATION_NETWORK_TRAFFIC_ANNOTATION_H_
+
+namespace net {
+
+// Defined type for network traffic annotation tags.
+using NetworkTrafficAnnotationTag = const char*;
+
+// Function to convert a network traffic annotation's unique id and protobuf
+// text into a NetworkTrafficAnnotationTag.
+//
+// This function serves as a tag that can be discovered and extracted via
+// clang tools. This allows reviewing all network traffic that is generated
+// and annotated by Chrome.
+//
+// |unique_id| should be a string that uniquely identifies this annotation
+// across all of Chromium source code.
+// |proto| is a text-encoded NetworkTrafficAnnotation protobuf (see
+// tools/traffic_annotaiton/traffic_annotation.proto)
+//
+// This function should be called with inlined parameters like
+//    NetworkTrafficAnnotationTag tag = DefineNetworkTrafficAnnotation(
+//        "unique_tag_id",
+//        R"(semantics: {...}
+//           policy: {...}
+//        )");
+// This allows the compiler to extract the |unique_id| at compile time without
+// copying the entire protobuf into the text segment of the binary or creating
+// any runtime performance impact.
+//
+// Please do NOT use the following syntax:
+//   const char* proto = R("...");
+//   NetworkTrafficAnnotationTag tag = DefineNetworkTrafficAnnotation(
+//       "unique_tag_id", proto);
+// You can see a sample and an empty template for text-coded protobuf in
+// (tools/traffic_annotation/sample_traffic_annotation.cc)
+// TODO(crbug.com/690323): Add tools to check annotation text's format during
+// presubmit checks.
+constexpr NetworkTrafficAnnotationTag DefineNetworkTrafficAnnotation(
+    const char* unique_id,
+    const char* proto) {
+  return (NetworkTrafficAnnotationTag)unique_id;
+}
+
+}  // namespace net
+
+// Placeholder for unannotated usages.
+#define NO_TRAFFIC_ANNOTATION_YET \
+  net::DefineNetworkTrafficAnnotation("Undefined", "Nothing here yet.")
+
+#endif  // NET_TRAFFIC_ANNOTATION_NETWORK_TRAFFIC_ANNOTATION_H_