Tracing V2: Trace buffer writer uses proto.

components/tracing/proto/events_chunk.proto

 // Copyright 2016 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.
 
 syntax = "proto2";
 option optimize_for = LITE_RUNTIME;
 
 package tracing.proto;
 
 // An EventsChunk is a fixed portion of the trace ring buffer which is written
 // exclusively by one writer (almost always 1 writer == 1 thread). It contains
 // a variable number of variable-sized events. Events can be fragmented across
 // several chunks.
+
 message EventsChunk {
   // Chunks produced by the same trace writer have the same writer_id. This
   // allows to identify the next chunk for a writer (thread) in the ring buffer.
   optional uint32 writer_id = 1;
 
   // A monotonic counter within the writer, to establish sequencing of chunks.
   optional uint32 seq_id = 2;
 
   // Each event is a byte sequence that encapsulates the bytes for a single
   // Event (see event.proto) or an incomplete fragment.
   // Why this isn't just a "repeated Events" field? An Event can spawn across
   // several chunks. In this case the reader has to first glue the byte
   // sequences together and then process the event.
   repeated bytes events = 3;
 
   // The first entry of |events| is a fragment which continues from the previous
   // chunk.
   optional bool first_event_continues_from_prev_chunk = 4;
 
   // The last entry of |events| is a fragment and contiues into the next chunk.
   optional bool last_event_continues_on_next_chunk = 5;
 }