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

components/tracing/BUILD.gn

Index: components/tracing/BUILD.gn
diff --git a/components/tracing/BUILD.gn b/components/tracing/BUILD.gn
index 612e88a6e23fd27b38d3cdabfe26643de29ec79f..a347f052a43e6b00be3dee2cf7defbc19155f799 100644
--- a/components/tracing/BUILD.gn
+++ b/components/tracing/BUILD.gn
@@ -23,6 +23,8 @@ component("tracing") {
     "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",
@@ -87,12 +89,14 @@ source_set("unit_tests") {
     "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",
@@ -106,3 +110,13 @@ source_set("unit_tests") {
     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",
+  ]
+}