base: Mark MessageLoop::Post*Task API deprecated

This is the first step in removing the direct MessageLoop task posting
API in favor of task_runner() and the TaskRunner APIs. See the design
doc for details[1].

BUG=465354

[1] https://docs.google.com/a/chromium.org/document/d/1qxdh2I61_aB_Uzh1QgNqvdWFBCL_E65G2smoSySw7KU/edit#heading=h.eqrpa9q6bsq5

Review URL: https://codereview.chromium.org/1013813003

Cr-Commit-Position: refs/heads/master@{#321674}


CrOS-Libchrome-Original-Commit: c8c18e9f1d3ac1033f9ca308143fc3b545475ebe
diff --git a/base/message_loop/message_loop.h b/base/message_loop/message_loop.h
index 7c76616..fd7596a 100644
--- a/base/message_loop/message_loop.h
+++ b/base/message_loop/message_loop.h
@@ -155,6 +155,9 @@
   // DestructionObserver is receiving a notification callback.
   void RemoveDestructionObserver(DestructionObserver* destruction_observer);
 
+  // NOTE: Deprecated; prefer task_runner() and the TaskRunner interfaces.
+  // TODO(skyostil): Remove these functions (crbug.com/465354).
+  //
   // The "PostTask" family of methods call the task's Run method asynchronously
   // from within a message loop at some point in the future.
   //
@@ -300,6 +303,8 @@
   }
 
   // Gets the TaskRunner associated with this message loop.
+  // TODO(skyostil): Change this to return a const reference to a refptr
+  // once the internal type matches what is being returned (crbug.com/465354).
   scoped_refptr<SingleThreadTaskRunner> task_runner() {
     return message_loop_proxy_;
   }