Revert of Adding task runner/ message loop to tests that use IsolateHolder.

gin/modules/module_registry_unittest.cc

Index: gin/modules/module_registry_unittest.cc
diff --git a/gin/modules/module_registry_unittest.cc b/gin/modules/module_registry_unittest.cc
index 00c6a94e1939ae4936245850daebe7c2f1ba019f..2d6e6daf83eb1eba0b76c5e93f2f2a1a4dea7db7 100644
--- a/gin/modules/module_registry_unittest.cc
+++ b/gin/modules/module_registry_unittest.cc
@@ -5,6 +5,7 @@
 #include "gin/modules/module_registry.h"
 
 #include "base/bind.h"
+#include "base/message_loop/message_loop.h"
 #include "gin/modules/module_registry_observer.h"
 #include "gin/modules/module_runner_delegate.h"
 #include "gin/public/context_holder.h"
@@ -24,6 +25,7 @@
         scope(runner.get()) {
   }
 
+  base::MessageLoop message_loop;
   ModuleRunnerDelegate delegate;
   scoped_ptr<ShellRunner> runner;
   Runner::Scope scope;