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

components/tracing/core/proto_zero_message.h

 // 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.
 
 #ifndef COMPONENTS_TRACING_CORE_PROTO_ZERO_MESSAGE_H_
 #define COMPONENTS_TRACING_CORE_PROTO_ZERO_MESSAGE_H_
 
 #include <stdint.h>
 
 #include <type_traits>
@@ skipping 12 matching lines @@
 namespace v2 {
 
 class ProtoZeroMessageHandleBase;
 
 // Base class extended by the proto C++ stubs generated by the ProtoZero
 // compiler (see //components/tracing/tools/). This class provides the minimal
 // runtime required to support append-only operations and is desiged for
 // performance. None of the methods require any dynamic memory allocation.
 class TRACING_EXPORT ProtoZeroMessage {
  public:
+  ProtoZeroMessage();
+
   // Clears up the state, allowing the message to be reused as a fresh one.
   void Reset(ScatteredStreamWriter*);
 
   // Commits all the changes to the buffer (backfills the size field of this and
   // all nested messages) and seals the message. Returns the size of the message
   // (and all nested sub-messages), without taking into account any chunking.
   // Finalize is idempotent and can be called several times w/o side effects.
   size_t Finalize();
 
   // Optional. If is_valid() == true, the corresponding memory region (its
@@ skipping 10 matching lines @@
   // message which is split into multiple chunks. Upon finalization only the
   // partial size that lies in the last chunk has to be backfilled.
   void inc_size_already_written(size_t size) { size_already_written_ += size; }
 
 #if DCHECK_IS_ON()
   // Only for ProtoZeroMessageHandleBase.
   void set_handle(ProtoZeroMessageHandleBase* handle) { handle_ = handle; }
 #endif
 
  protected:
-  ProtoZeroMessage();
-
   // Proto types: uint64, uint32, int64, int32, bool, enum.
   template <typename T>
   void AppendVarInt(uint32_t field_id, T value) {
     if (nested_message_)
       EndNestedMessage();
 
     uint8_t buffer[proto::kMaxSimpleFieldEncodedSize];
     uint8_t* pos = buffer;
 
     pos = proto::WriteVarInt(proto::MakeTagVarInt(field_id), pos);
@@ skipping 115 matching lines @@
   // DO NOT add any fields below |nested_messages_arena_|. The memory layout of
   // nested messages would overflow the storage allocated by the root message.
 
   DISALLOW_COPY_AND_ASSIGN(ProtoZeroMessage);
 };
 
 }  // namespace v2
 }  // namespace tracing
 
 #endif  // COMPONENTS_TRACING_CORE_PROTO_ZERO_MESSAGE_H_