gn: Support build directories outside the source tree.

tools/gn/function_set_default_toolchain.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/build_settings.h"
 #include "tools/gn/functions.h"
 #include "tools/gn/loader.h"
 #include "tools/gn/parse_tree.h"
 #include "tools/gn/scope.h"
 #include "tools/gn/settings.h"
@@ skipping 49 matching lines @@
       scope->GetProperty(Loader::kDefaultToolchainKey, NULL));
   if (!default_toolchain_dest)
     return Value();
 
   const SourceDir& current_dir = scope->GetSourceDir();
   const Label& default_toolchain = ToolchainLabelForScope(scope);
 
   if (!EnsureSingleStringArg(function, args, err))
     return Value();
   Label toolchain_label(
-      Label::Resolve(current_dir, default_toolchain, args[0], err));
+      Label::Resolve(current_dir, default_toolchain, args[0],
+                     scope->settings()->build_settings()->root_path(), err));
   if (toolchain_label.is_null())
     return Value();
 
   *default_toolchain_dest = toolchain_label;
   return Value();
 }
 
 }  // namespace functions