Hook up link pools in GN.

tools/gn/toolchain.h

 // 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.
 
 #ifndef TOOLS_GN_TOOLCHAIN_H_
 #define TOOLS_GN_TOOLCHAIN_H_
 
 #include "base/compiler_specific.h"
 #include "base/logging.h"
 #include "base/memory/scoped_ptr.h"
@@ skipping 90 matching lines @@
   // will return the stamp tool ionstead since the final output of a copy
   // target is to stamp the set of copies done so there is one output.
   static ToolType GetToolTypeForTargetFinalOutput(const Target* target);
   const Tool* GetToolForTargetFinalOutput(const Target* target) const;
 
   const SubstitutionBits& substitution_bits() const {
     DCHECK(setup_complete_);
     return substitution_bits_;
   }
 
+  void set_concurrent_links(int cl) { concurrent_links_ = cl; }
+  int concurrent_links() const { return concurrent_links_; }
+
  private:
   scoped_ptr<Tool> tools_[TYPE_NUMTYPES];
 
+  // How many links to run in parallel. Only the default toolchain's version of
+  // this variable applies.
+  int concurrent_links_;
+
   bool setup_complete_;
 
   // Substitutions used by the tools in this toolchain.
   SubstitutionBits substitution_bits_;
 
   LabelTargetVector deps_;
   Scope::KeyValueMap args_;
 };
 
 #endif  // TOOLS_GN_TOOLCHAIN_H_