Revert of base: Remove use of MessageLoopProxy

base/test/thread_test_helper.cc

Index: base/test/thread_test_helper.cc
diff --git a/base/test/thread_test_helper.cc b/base/test/thread_test_helper.cc
index 6a12190898fab773d9c481df7b9b65ad20a0236f..2bd3ba54ac2e98a51cb1a5fce4d79eb37c5fff2b 100644
--- a/base/test/thread_test_helper.cc
+++ b/base/test/thread_test_helper.cc
@@ -11,9 +11,9 @@
 namespace base {
 
 ThreadTestHelper::ThreadTestHelper(
-    scoped_refptr<SingleThreadTaskRunner> target_thread)
+    const scoped_refptr<MessageLoopProxy>& target_thread)
     : test_result_(false),
-      target_thread_(target_thread.Pass()),
+      target_thread_(target_thread),
       done_event_(false, false) {
 }