Add support for user defined "pool" to GN.

tools/gn/item.h

Index: tools/gn/item.h
diff --git a/tools/gn/item.h b/tools/gn/item.h
index 069c3edc55848419f636171ac08215b23eb12e45..3ec482af1df7b11445f07eee1dcb139f48c28ae9 100644
--- a/tools/gn/item.h
+++ b/tools/gn/item.h
@@ -12,6 +12,7 @@
 
 class Config;
 class ParseNode;
+class Pool;
 class Settings;
 class Target;
 class Toolchain;
@@ -38,6 +39,8 @@ class Item {
   // Manual RTTI.
   virtual Config* AsConfig();
   virtual const Config* AsConfig() const;
+  virtual Pool* AsPool();
+  virtual const Pool* AsPool() const;
   virtual Target* AsTarget();
   virtual const Target* AsTarget() const;
   virtual Toolchain* AsToolchain();