Revert "gn: Remove Scheduler::pool()."

tools/gn/scheduler.h

Index: tools/gn/scheduler.h
diff --git a/tools/gn/scheduler.h b/tools/gn/scheduler.h
index e20bcedef25323a558464726b1ef93dd5ef85565..67650d8fbcf804b129dc613669fbe3c83759c9fe 100644
--- a/tools/gn/scheduler.h
+++ b/tools/gn/scheduler.h
@@ -33,6 +33,7 @@ class Scheduler {
   scoped_refptr<base::SingleThreadTaskRunner> task_runner() {
     return main_loop_.task_runner();
   }
+  base::SequencedWorkerPool* pool() { return pool_.get(); }
 
   InputFileManager* input_file_manager() { return input_file_manager_.get(); }