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

components/tracing/core/proto_zero_message.h

Index: components/tracing/core/proto_zero_message.h
diff --git a/components/tracing/core/proto_zero_message.h b/components/tracing/core/proto_zero_message.h
index b5b0a37cce1bc4b82788c62108ee7036702cb6d0..6723eb6aeab88f619ad8cd473acc7b81156bda44 100644
--- a/components/tracing/core/proto_zero_message.h
+++ b/components/tracing/core/proto_zero_message.h
@@ -30,6 +30,8 @@ class ProtoZeroMessageHandleBase;
 // 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*);
 
@@ -60,8 +62,6 @@ class TRACING_EXPORT ProtoZeroMessage {
 #endif
 
  protected:
-  ProtoZeroMessage();
-
   // Proto types: uint64, uint32, int64, int32, bool, enum.
   template <typename T>
   void AppendVarInt(uint32_t field_id, T value) {