Remove calls to MessageLoop::current() in tools.

tools/gn/scheduler.h

Index: tools/gn/scheduler.h
diff --git a/tools/gn/scheduler.h b/tools/gn/scheduler.h
index a9bb036512259af6622f24b52997d484b24effb8..67650d8fbcf804b129dc613669fbe3c83759c9fe 100644
--- a/tools/gn/scheduler.h
+++ b/tools/gn/scheduler.h
@@ -12,6 +12,7 @@
 #include "base/macros.h"
 #include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
+#include "base/single_thread_task_runner.h"
 #include "base/synchronization/lock.h"
 #include "base/threading/sequenced_worker_pool.h"
 #include "tools/gn/input_file_manager.h"
@@ -29,7 +30,9 @@ class Scheduler {
 
   bool Run();
 
-  base::MessageLoop* main_loop() { return &main_loop_; }
+  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(); }