Hook up link pools in GN.

tools/gn/ninja_build_writer.h

Index: tools/gn/ninja_build_writer.h
diff --git a/tools/gn/ninja_build_writer.h b/tools/gn/ninja_build_writer.h
index b8c87421a827975b468b7958265e533ef1e25d44..1de6719ede224bc204dad3840380ed83a1cdfcaf 100644
--- a/tools/gn/ninja_build_writer.h
+++ b/tools/gn/ninja_build_writer.h
@@ -13,6 +13,7 @@
 class BuildSettings;
 class Settings;
 class Target;
+class Toolchain;
 
 // Generates the toplevel "build.ninja" file. This references the individual
 // toolchain files and lists all input .gn files as dependencies of the
@@ -22,11 +23,13 @@ class NinjaBuildWriter {
   static bool RunAndWriteFile(
       const BuildSettings* settings,
       const std::vector<const Settings*>& all_settings,
+      const Toolchain* default_toolchain,
       const std::vector<const Target*>& default_toolchain_targets);
 
  private:
   NinjaBuildWriter(const BuildSettings* settings,
                    const std::vector<const Settings*>& all_settings,
+                   const Toolchain* default_toolchain,
                    const std::vector<const Target*>& default_toolchain_targets,
                    std::ostream& out,
                    std::ostream& dep_out);
@@ -35,6 +38,7 @@ class NinjaBuildWriter {
   void Run();
 
   void WriteNinjaRules();
+  void WriteLinkPool();
   void WriteSubninjas();
   void WritePhonyAndAllRules();
 
@@ -43,6 +47,7 @@ class NinjaBuildWriter {
 
   const BuildSettings* build_settings_;
   std::vector<const Settings*> all_settings_;
+  const Toolchain* default_toolchain_;
   std::vector<const Target*> default_toolchain_targets_;
   std::ostream& out_;
   std::ostream& dep_out_;