Revert of base: Remove use of MessageLoopProxy (patchset #6 id:100001 of https://codereview.chromium.org/1100773004/)

Reason for revert:
This CL caused this failure:
http://build.chromium.org/p/chromium.chromiumos/builders/Linux%20ChromiumOS%20Tests%20%281%29/builds/2126

I would strongly recommend doing this in smaller pieces since it combines mechanical changes with more subtle ones (base/prefs, base/task).

Original issue's description:
> base: Remove use of MessageLoopProxy
>
> Replace usage of MessageLoopProxy under base/ with SingleThreadTaskRunner
> and ThreadTaskRunnerHandle (excluding the implementation of MessageLoopProxy
> itself which will removed later).
>
> This patch was mostly autogenerated with
> https://codereview.chromium.org/1010073002.
>
> Depends on https://codereview.chromium.org/1086733002/.
>
> BUG=465354
> TBR=nkostylev@chromium.org,pkasting@chromium.org,pauljensen@chromium.org
>
> Committed: https://crrev.com/62aa5ca413e15738ebebbb9acd271138ec808739
> Cr-Commit-Position: refs/heads/master@{#327512}

TBR=danakj@chromium.org,skyostil@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=465354

Review URL: https://codereview.chromium.org/1113953002

Cr-Commit-Position: refs/heads/master@{#327573}


CrOS-Libchrome-Original-Commit: 7669182866d8605d98a5141f7b82be3b516799ca
diff --git a/base/files/file_path_watcher.h b/base/files/file_path_watcher.h
index 4f132af..d26fa06 100644
--- a/base/files/file_path_watcher.h
+++ b/base/files/file_path_watcher.h
@@ -12,7 +12,7 @@
 #include "base/callback.h"
 #include "base/files/file_path.h"
 #include "base/memory/ref_counted.h"
-#include "base/single_thread_task_runner.h"
+#include "base/message_loop/message_loop_proxy.h"
 
 namespace base {
 
@@ -58,13 +58,12 @@
     // check |is_cancelled()| to avoid duplicate work.
     virtual void CancelOnMessageLoopThread() = 0;
 
-    scoped_refptr<base::SingleThreadTaskRunner> task_runner() const {
-      return task_runner_;
+    scoped_refptr<base::MessageLoopProxy> message_loop() const {
+      return message_loop_;
     }
 
-    void set_task_runner(
-        scoped_refptr<base::SingleThreadTaskRunner> task_runner) {
-      task_runner_ = task_runner.Pass();
+    void set_message_loop(const scoped_refptr<base::MessageLoopProxy>& loop) {
+      message_loop_ = loop;
     }
 
     // Must be called before the PlatformDelegate is deleted.
@@ -77,7 +76,7 @@
     }
 
    private:
-    scoped_refptr<base::SingleThreadTaskRunner> task_runner_;
+    scoped_refptr<base::MessageLoopProxy> message_loop_;
     bool cancelled_;
   };