Hook up link pools in GN.

tools/gn/toolchain.h

Index: tools/gn/toolchain.h
diff --git a/tools/gn/toolchain.h b/tools/gn/toolchain.h
index 8264577e0b0c367cd6ff90d268f3d20cce3b4a93..393596cb2917f7d11832f0412307cca078939214 100644
--- a/tools/gn/toolchain.h
+++ b/tools/gn/toolchain.h
@@ -108,9 +108,16 @@ class Toolchain : public Item {
     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.