Introduce task_runner() accessors for both base::Thread and base::MessageLoop

base/message_loop/message_loop.h

Index: base/message_loop/message_loop.h
diff --git a/base/message_loop/message_loop.h b/base/message_loop/message_loop.h
index a65fbf5a760506f121c2a98bc67a083a2768d59d..330bde6d2e8da68dccf0c1af1fcfaf2a8d081138 100644
--- a/base/message_loop/message_loop.h
+++ b/base/message_loop/message_loop.h
@@ -295,10 +295,17 @@ class BASE_EXPORT MessageLoop : public MessagePump::Delegate {
   const std::string& thread_name() const { return thread_name_; }
 
   // Gets the message loop proxy associated with this message loop.
+  //
+  // NOTE: Deprecated; prefer task_runner() and the TaskRunner interfaces
   scoped_refptr<MessageLoopProxy> message_loop_proxy() {
     return message_loop_proxy_;
   }
 
+  // Gets the TaskRunner associated with this message loop.
+  scoped_refptr<SingleThreadTaskRunner> task_runner() {
+    return message_loop_proxy_;
+  }
+
   // Enables or disables the recursive task processing. This happens in the case
   // of recursive message loops. Some unwanted message loop may occurs when
   // using common controls or printer functions. By default, recursive task