base: Remove most uses of MessageLoopProxy

base/single_thread_task_runner.h

Index: base/single_thread_task_runner.h
diff --git a/base/single_thread_task_runner.h b/base/single_thread_task_runner.h
index b5311db16942f1fe7d791ea454104cc3eccbadb4..6e9319314894c3f8ed52269c9e526c96fa92e6d5 100644
--- a/base/single_thread_task_runner.h
+++ b/base/single_thread_task_runner.h
@@ -16,7 +16,8 @@ namespace base {
 // there is a specific need to run tasks on only a single thread.
 //
 // SingleThreadTaskRunner implementations might:
-//   - Post tasks to an existing thread's MessageLoop (see MessageLoopProxy).
+//   - Post tasks to an existing thread's MessageLoop (see
+//     MessageLoop::task_runner()).
 //   - Create their own worker thread and MessageLoop to post tasks to.
 //   - Add tasks to a FIFO and signal to a non-MessageLoop thread for them to
 //     be processed. This allows TaskRunner-oriented code run on threads