GN threading refactor

tools/gn/settings.cc

 // Copyright (c) 2013 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/settings.h"
 
 #include "base/logging.h"
 #include "build/build_config.h"
 #include "tools/gn/filesystem_utils.h"
 
 Settings::Settings(const BuildSettings* build_settings,
                    const std::string& output_subdir_name)
     : build_settings_(build_settings),
-      is_default_(false),
       import_manager_(),
       base_config_(this),
       greedy_target_generation_(false) {
   DCHECK(output_subdir_name.find('/') == std::string::npos);
   if (output_subdir_name.empty()) {
     toolchain_output_dir_ = build_settings->build_dir();
   } else {
     // We guarantee this ends in a slash.
     toolchain_output_subdir_.value().append(output_subdir_name);
     toolchain_output_subdir_.value().push_back('/');
 
+    DCHECK(!build_settings->build_dir().is_null());
     toolchain_output_dir_ = SourceDir(build_settings->build_dir().value() +
                                       toolchain_output_subdir_.value());
   }
   // The output dir will be null in some tests and when invoked to parsed
   // one-off data without doing generation.
   if (!toolchain_output_dir_.is_null())
     toolchain_gen_dir_ = SourceDir(toolchain_output_dir_.value() + "gen/");
 
 #if defined(OS_WIN)
   target_os_ = WIN;
 #elif defined(OS_MACOSX)
   target_os_ = MAC;
 #elif defined(OS_LINUX)
   target_os_ = LINUX;
 #else
   #error implement me
 #endif
 }
 
 Settings::~Settings() {
 }
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