Revert of base: Remove use of MessageLoopProxy

base/observer_list_threadsafe.h

Index: base/observer_list_threadsafe.h
diff --git a/base/observer_list_threadsafe.h b/base/observer_list_threadsafe.h
index e0ce0daa90f74e5db7ddef6bb250f70f35989c61..5205a5a9d1f47f4f696592195269c290cf874e1f 100644
--- a/base/observer_list_threadsafe.h
+++ b/base/observer_list_threadsafe.h
@@ -14,10 +14,9 @@
 #include "base/logging.h"
 #include "base/memory/ref_counted.h"
 #include "base/message_loop/message_loop.h"
+#include "base/message_loop/message_loop_proxy.h"
 #include "base/observer_list.h"
-#include "base/single_thread_task_runner.h"
 #include "base/stl_util.h"
-#include "base/thread_task_runner_handle.h"
 #include "base/threading/platform_thread.h"
 
 ///////////////////////////////////////////////////////////////////////////////
@@ -178,7 +177,7 @@
     base::AutoLock lock(list_lock_);
     for (const auto& entry : observer_lists_) {
       ObserverListContext* context = entry.second;
-      context->task_runner->PostTask(
+      context->loop->PostTask(
           from_here,
           base::Bind(
               &ObserverListThreadSafe<ObserverType>::template NotifyWrapper<
@@ -193,9 +192,11 @@
 
   struct ObserverListContext {
     explicit ObserverListContext(NotificationType type)
-        : task_runner(base::ThreadTaskRunnerHandle::Get()), list(type) {}
-
-    scoped_refptr<base::SingleThreadTaskRunner> task_runner;
+        : loop(base::MessageLoopProxy::current()),
+          list(type) {
+    }
+
+    scoped_refptr<base::MessageLoopProxy> loop;
     ObserverList<ObserverType> list;
 
    private: