Use $root:default as a "default" rule

tools/gn/setup.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/setup.h"
 
 #include <stdlib.h>
 #include <algorithm>
 #include <sstream>
 #include <utility>
@@ skipping 680 matching lines @@
     err.PrintToStdout();
     return false;
   }
 
   return true;
 }
 
 bool Setup::FillOtherConfig(const base::CommandLine& cmdline) {
   Err err;
   SourceDir current_dir("//");
+  Label root_target_label(current_dir, "");
 
   // Secondary source path, read from the config file if present.
   // Read from the config file if present.
   const Value* secondary_value =
       dotfile_scope_.GetValue("secondary_source", true);
   if (secondary_value) {
     if (!secondary_value->VerifyTypeIs(Value::STRING, &err)) {
       err.PrintToStdout();
       return false;
     }
     build_settings_.SetSecondarySourcePath(
         SourceDir(secondary_value->string_value()));
   }
 
   // Root build file.
   const Value* root_value = dotfile_scope_.GetValue("root", true);
   if (root_value) {
     if (!root_value->VerifyTypeIs(Value::STRING, &err)) {
       err.PrintToStdout();
       return false;
     }
 
-    Label root_target_label =
-        Label::Resolve(current_dir, Label(), *root_value, &err);
+    root_target_label = Label::Resolve(current_dir, Label(), *root_value, &err);
     if (err.has_error()) {
       err.PrintToStdout();
       return false;
     }
 
     root_build_file_ = Loader::BuildFileForLabel(root_target_label);
   }
+  build_settings_.SetRootTargetLabel(root_target_label);
 
   // Build config file.
   const Value* build_config_value =
       dotfile_scope_.GetValue("buildconfig", true);
   if (!build_config_value) {
     Err(Location(), "No build config file.",
         "Your .gn file (\"" + FilePathToUTF8(dotfile_name_) + "\")\n"
         "didn't specify a \"buildconfig\" value.").PrintToStdout();
     return false;
   } else if (!build_config_value->VerifyTypeIs(Value::STRING, &err)) {
@@ skipping 58 matching lines @@
     if (!arg_file_template_value->VerifyTypeIs(Value::STRING, &err)) {
       err.PrintToStdout();
       return false;
     }
     SourceFile path(arg_file_template_value->string_value());
     build_settings_.set_arg_file_template_path(path);
   }
 
   return true;
 }