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

components/tracing/core/proto_zero_message_handle.cc

 // 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.
 
 #include "components/tracing/core/proto_zero_message_handle.h"
 
 #include "base/logging.h"
 #include "components/tracing/core/proto_zero_message.h"
 
 namespace tracing {
@@ skipping 24 matching lines @@
   FinalizeMessageIfSet(message_);
 }
 
 ProtoZeroMessageHandleBase::ProtoZeroMessageHandleBase(
     ProtoZeroMessageHandleBase&& other) {
   Move(&other);
 }
 
 ProtoZeroMessageHandleBase& ProtoZeroMessageHandleBase::operator=(
     ProtoZeroMessageHandleBase&& other) {
+  // If the current handle was pointing to a message and is being reset to a new
+  // one, finalize the old message.
+  FinalizeMessageIfSet(message_);
+
   Move(&other);
   return *this;
 }
 
 void ProtoZeroMessageHandleBase::Move(ProtoZeroMessageHandleBase* other) {
-  // If the current handle was pointing to a message and is being reset to a new
-  // one, finalize the old message.
-  FinalizeMessageIfSet(message_);
-
   // In theory other->message_ could be nullptr, if |other| is a handle that has
   // been std::move-d (and hence empty). There isn't a legitimate use case for
   // doing so, though. Therefore this case is deliberately ignored (if hit, it
   // will manifest as a segfault when dereferencing |message_| below) to avoid a
   // useless null-check.
   message_ = other->message_;
   other->message_ = nullptr;
 #if DCHECK_IS_ON()
   message_->set_handle(this);
 #endif
 }
 
 }  // namespace v2
 }  // namespace tracing