Hook up link pools in GN.

tools/gn/ninja_writer.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/ninja_writer.h"
 
 #include "tools/gn/builder.h"
 #include "tools/gn/loader.h"
 #include "tools/gn/location.h"
 #include "tools/gn/ninja_build_writer.h"
 #include "tools/gn/ninja_toolchain_writer.h"
+#include "tools/gn/settings.h"
 
 NinjaWriter::NinjaWriter(const BuildSettings* build_settings,
                          Builder* builder)
     : build_settings_(build_settings),
       builder_(builder) {
 }
 
 NinjaWriter::~NinjaWriter() {
 }
 
@@ skipping 59 matching lines @@
     }
   }
 
   *default_targets = categorized[default_label];
   return true;
 }
 
 bool NinjaWriter::WriteRootBuildfiles(
     const std::vector<const Settings*>& all_settings,
     const std::vector<const Target*>& default_targets) {
+  // All Settings objects should have the same default toolchain, and there
+  // should always be at least one settings object in the build.
+  CHECK(!all_settings.empty());
+  const Toolchain* default_toolchain =
+      builder_->GetToolchain(all_settings[0]->default_toolchain_label());
+
   // Write the root buildfile.
   if (!NinjaBuildWriter::RunAndWriteFile(build_settings_, all_settings,
-                                         default_targets)) {
+                                         default_toolchain, default_targets)) {
     Err(Location(),
         "Couldn't open toolchain buildfile(s) for writing").PrintToStdout();
     return false;
   }
   return true;
 }