GN threading refactor

tools/gn/settings.cc

Index: tools/gn/settings.cc
diff --git a/tools/gn/settings.cc b/tools/gn/settings.cc
index b8de9766fe97e07a8fe7eff4e5079c3ec720b2d6..2715829998b71202624b3ba381d9b197f9d43c22 100644
--- a/tools/gn/settings.cc
+++ b/tools/gn/settings.cc
@@ -11,7 +11,6 @@
 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) {
@@ -23,6 +22,7 @@ Settings::Settings(const BuildSettings* build_settings,
     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());
   }
@@ -44,5 +44,3 @@ Settings::Settings(const BuildSettings* build_settings,
 
 Settings::~Settings() {
 }
-
-