Allow paths to be resolved in the GN args file

This uses the real settings object which contains the location of the
source root for resolving "//..." paths. The use-case is to allow
imports to be processed in the args file, which in turn allows one to assign
a set of build arguments by reference.

Review URL: https://codereview.chromium.org/1104313002

Cr-Commit-Position: refs/heads/master@{#327131}

tools/gn/setup.cc

@@ -160,10 +160,10 @@ Setup::Setup()
       check_for_bad_items_(true),
       check_for_unused_overrides_(true),
       check_public_headers_(false),
-      empty_settings_(&empty_build_settings_, std::string()),
-      dotfile_scope_(&empty_settings_),
+      dotfile_settings_(&build_settings_, std::string()),
+      dotfile_scope_(&dotfile_settings_),
       fill_arguments_(true) {
-  empty_settings_.set_toolchain_label(Label());
+  dotfile_settings_.set_toolchain_label(Label());
   build_settings_.set_item_defined_callback(
       base::Bind(&ItemDefinedCallback, scheduler_.main_loop(), builder_));
 
@@ -340,7 +340,7 @@ bool Setup::FillArgsFromArgsInputFile() {
     return false;
   }
 
-  Scope arg_scope(&empty_settings_);
+  Scope arg_scope(&dotfile_settings_);
   args_root_->Execute(&arg_scope, &err);
   if (err.has_error()) {
     err.PrintToStdout();

tools/gn/setup.h

@@ -151,10 +151,9 @@ class Setup {
 
   Scheduler scheduler_;
 
-  // These empty settings and toolchain are used to interpret the command line
-  // and dot file.
-  BuildSettings empty_build_settings_;
-  Settings empty_settings_;
+  // These settings and toolchain are used to interpret the command line and
+  // dot file.
+  Settings dotfile_settings_;
   Scope dotfile_scope_;
 
   // State for invoking the dotfile.