Revert of Protect MessageQueue stop field with a critical section to avoid data races. (patchset #5 id:80001 of https://codereview.webrtc.org/2023193002/ )

Reason for revert:
Only reasonable CL in blameslist for broken Chrome FYI bots on all platforms. See

https://build.chromium.org/p/chromium.webrtc.fyi/waterfall?builder=Mac%20Builder

Original issue's description:
> Protect MessageQueue stop field with a critical section to avoid data races.
>
> Committed: https://crrev.com/1d35d2971b1e89b3ecadb7fb1ff064f9af850ad4
> Cr-Commit-Position: refs/heads/master@{#13430}

TBR=pthatcher@webrtc.org,tommi@webrtc.org,deadbeef@webrtc.org,tommi@chromium.org,pbos@webrtc.org,andresp@webrtc.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true

Review-Url: https://codereview.webrtc.org/2135173002
Cr-Commit-Position: refs/heads/master@{#13431}
diff --git a/webrtc/base/messagequeue.h b/webrtc/base/messagequeue.h
index 628ce44..a1b20da 100644
--- a/webrtc/base/messagequeue.h
+++ b/webrtc/base/messagequeue.h
@@ -288,6 +288,7 @@
 
   void WakeUpSocketServer();
 
+  bool fStop_;
   bool fPeekKeep_;
   Message msgPeek_;
   MessageList msgq_ GUARDED_BY(crit_);
@@ -298,8 +299,6 @@
   bool fDestroyed_;
 
  private:
-  volatile int stop_;
-
   // The SocketServer might not be owned by MessageQueue.
   SocketServer* ss_ GUARDED_BY(ss_lock_);
   // Used if SocketServer ownership lies with |this|.