Add support for user defined "pool" to GN.

tools/gn/toolchain.cc

Index: tools/gn/toolchain.cc
diff --git a/tools/gn/toolchain.cc b/tools/gn/toolchain.cc
index 14d2bfd159590bb0775d5b081f2ebb42e8f5779e..03c9c59a843ca825be9d91bf84500732dd695b73 100644
--- a/tools/gn/toolchain.cc
+++ b/tools/gn/toolchain.cc
@@ -86,6 +86,11 @@ std::string Toolchain::ToolTypeToName(ToolType type) {
   }
 }
 
+Tool* Toolchain::GetTool(ToolType type) {
+  DCHECK(type != TYPE_NONE);
+  return tools_[static_cast<size_t>(type)].get();
+}
+
 const Tool* Toolchain::GetTool(ToolType type) const {
   DCHECK(type != TYPE_NONE);
   return tools_[static_cast<size_t>(type)].get();