Remove calls to deprecated MessageLoop methods in tools.

tools/gn/setup.cc

Index: tools/gn/setup.cc
diff --git a/tools/gn/setup.cc b/tools/gn/setup.cc
index ea7cb240908b701dd1d2ebf332adf09540505519..5796ada203ea4e384bc1aa597016929d25d96ace 100644
--- a/tools/gn/setup.cc
+++ b/tools/gn/setup.cc
@@ -14,6 +14,7 @@
 #include "base/files/file_path.h"
 #include "base/files/file_util.h"
 #include "base/process/launch.h"
+#include "base/single_thread_task_runner.h"
 #include "base/strings/string_split.h"
 #include "base/strings/string_util.h"
 #include "base/strings/sys_string_conversions.h"
@@ -142,8 +143,9 @@ void ItemDefinedCallback(base::MessageLoop* main_loop,
                          scoped_refptr<Builder> builder,
                          std::unique_ptr<Item> item) {
   DCHECK(item);
-  main_loop->PostTask(FROM_HERE, base::Bind(&Builder::ItemDefined, builder,
-                                            base::Passed(&item)));
+  main_loop->task_runner()->PostTask(
+      FROM_HERE,
+      base::Bind(&Builder::ItemDefined, builder, base::Passed(&item)));
 }
 
 void DecrementWorkCount() {