Reland of tracing v2: Introduce TraceBufferWriter (https://codereview.chromium.org/2196663002)

components/tracing/BUILD.gn

 # Copyright 2014 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.
 
 import("//third_party/protobuf/proto_library.gni")
 
 component("tracing") {
   sources = [
     "child/child_memory_dump_manager_delegate_impl.cc",
     "child/child_memory_dump_manager_delegate_impl.h",
     "child/child_trace_message_filter.cc",
     "child/child_trace_message_filter.h",
     "common/graphics_memory_dump_provider_android.cc",
     "common/graphics_memory_dump_provider_android.h",
     "common/process_metrics_memory_dump_provider.cc",
     "common/process_metrics_memory_dump_provider.h",
     "common/tracing_messages.cc",
     "common/tracing_messages.h",
     "core/proto_utils.h",
     "core/proto_zero_message.cc",
     "core/proto_zero_message.h",
     "core/proto_zero_message_handle.cc",
     "core/proto_zero_message_handle.h",
     "core/scattered_stream_writer.cc",
     "core/scattered_stream_writer.h",
+    "core/trace_buffer_writer.cc",
+    "core/trace_buffer_writer.h",
     "core/trace_ring_buffer.cc",
     "core/trace_ring_buffer.h",
     "tracing_export.h",
   ]
 
   defines = [ "TRACING_IMPLEMENTATION" ]
 
   deps = [
     "//base",
     "//ipc",
@@ skipping 44 matching lines @@
 source_set("unit_tests") {
   testonly = true
 
   sources = [
     "child/child_trace_message_filter_unittest.cc",
     "common/graphics_memory_dump_provider_android_unittest.cc",
     "common/process_metrics_memory_dump_provider_unittest.cc",
     "core/proto_utils_unittest.cc",
     "core/proto_zero_message_unittest.cc",
     "core/scattered_stream_writer_unittest.cc",
+    "core/trace_buffer_writer_unittest.cc",
     "core/trace_ring_buffer_unittest.cc",
     "test/fake_scattered_buffer.cc",
     "test/proto_zero_generation_unittest.cc",
   ]
 
   deps = [
+    ":golden_protos_for_tests",
     ":proto_zero_testing_messages",
     ":tracing",
     "//base/test:test_support",
     "//ipc",
     "//testing/gmock:gmock",
     "//testing/gtest",
   ]
 
   if (!is_android) {
     sources += [ "browser/trace_config_file_unittest.cc" ]
     deps += [ ":startup_tracing" ]
   }
 }
+
+# Official protobuf used by tests to verify that the Tracing V2 output is
+# effectively proto-compatible.
+proto_library("golden_protos_for_tests") {
+  visibility = [ ":unit_tests" ]
+  proto_out_dir = "components/tracing/test/golden_protos"
+  sources = [
+    "proto/events_chunk.proto",
+  ]
+}