GN: Initialize Tool::defined_from_ in the constructor

tools/gn/tool.cc

 // Copyright 2014 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 "tools/gn/tool.h"
 
 Tool::Tool()
-    : depsformat_(DEPS_GCC),
+    : defined_from_(nullptr),
+      depsformat_(DEPS_GCC),
       precompiled_header_type_(PCH_NONE),
       restat_(false),
       complete_(false) {
 }
 
 Tool::~Tool() {
 }
 
 void Tool::SetComplete() {
   DCHECK(!complete_);
   complete_ = true;
 
   command_.FillRequiredTypes(&substitution_bits_);
   depfile_.FillRequiredTypes(&substitution_bits_);
   description_.FillRequiredTypes(&substitution_bits_);
   outputs_.FillRequiredTypes(&substitution_bits_);
   link_output_.FillRequiredTypes(&substitution_bits_);
   depend_output_.FillRequiredTypes(&substitution_bits_);
   rspfile_.FillRequiredTypes(&substitution_bits_);
   rspfile_content_.FillRequiredTypes(&substitution_bits_);
 }